2.1 Discrete series

2.1.1 Parametrization

For this section, let G be a reductive group over $\mathbb R$ and K a maximal compact of $G(\mathbb R)$ . Assume G has elliptic torus T so that $G(\mathbb R)$ has discrete series. Without loss of generality, $T \subseteq K$ . Recall the notation from [Reference DalalDal22, §2.2.1] to discuss discrete series. In particular, recall the two parametrizations of discrete series on $G(\mathbb R)$ :

$$\begin{align*}\pi^G_{\lambda, \omega} = \pi^G_{\omega(\lambda + \rho_{G})} \end{align*}$$

for $\lambda $ a dominant (but possibly irregular) weight of T and $\omega $ a Weyl-element that takes a chosen $\Omega _G$ dominant chamber into a chosen $\Omega _K$ -dominant one. Note that $\pi ^G_{\lambda , \omega }$ has infinitesimal character $\lambda + \rho _{G}$ . Recall that $\omega (\lambda + \rho _{G})$ is called the Harish–Chandra parameter of this discrete series.

2.1.2 Their pseudocoefficients

Recall from the corollary on page 213 in [Reference Clozel and DelormeCD90] the notion of pseudocoefficients $\varphi _\pi $ for discrete series $\pi $ . They are defined by their traces against standard modules $\rho $ :

$$\begin{align*}\operatorname{\mathrm{tr}}_\rho(\varphi_\pi) = \begin{cases} 1 & \rho = \pi \\ 0 & \rho \text{ standard, } \sigma \neq \pi \end{cases}. \end{align*}$$

Here, a standard module is a parabolic induction of a discrete series or limit of discrete series.

Note. By the Langlands classification, every irreducible representation has a character formula writing it as a linear combination of standard modules in the Grothendieck group. By linearity of trace, if $\sigma $ is an irreducible representation, then $\operatorname {\mathrm {tr}}_\sigma (\varphi _\pi )$ is the coefficient of $\pi $ in its character formula.

Recall also the Euler–Poincaré functions $\eta _\lambda $ that we normalize to be the average of pseudocoefficients over an L-packet of infinitesimal character $\lambda + \rho _G$ . For a quick summary of relevant properties of these functions in the notation used here, see [Reference DalalDal22, §2.2.2].

2.2 Trace distinguishability

A priori, the trace against $\varphi _\pi $ may be nonzero for certain nontempered representations in addition to just $\pi $ . This could make $\varphi _k$ unusable as a test function to pick out just automorphic representations $\pi $ with $\pi _\infty = \pi $ . We analyze when this happens.

Definition. Call discrete series $\pi $ on group $G(\mathbb R)$ trace-distinguishable if for all unitary representations $\sigma $ of $G(\mathbb R)$

$$\begin{align*}\operatorname{\mathrm{tr}}_\sigma(\varphi_k) = \begin{cases} 1 & \sigma = \pi_k \\ 0 & \text{else} \end{cases}. \end{align*}$$

To motivate this definition, the Paley–Weiner theorem of [Reference Clozel and DelormeCD90] shows that $\varphi _\pi $ is the only compactly supported function that could have the property of isolating $\pi $ in the unitary dual in this way – there are none if $\pi $ isn’t trace-distinguishable.

Proposition 2.2.1. Let discrete series $\pi $ on $G(\mathbb R)$ have Harish–Chandra parameter $\xi $ . Define

$$\begin{align*}S_\xi = \{\alpha \in \Phi_G : \langle \xi, \alpha^\vee \rangle = 1\}, \end{align*}$$

where $\Phi _G$ is the set of roots of $(G, T)$ for T elliptic. Then $\pi $ is trace-distinguishable if and only if $\pi $ contains no noncompact roots.

Proof. The following proof was described to me by David Vogan. Choose simple roots so that $\xi $ is dominant. By the same argument of Vogan described in [Reference DalalDal22, lem. 6.3.1], $\operatorname {\mathrm {tr}}_\sigma (\varphi _\pi ) = 0$ unless $\operatorname {\mathrm {tr}}_\sigma (\eta _{\xi - \rho _G}) \neq 0$ for $\eta _{\xi - \rho _G}$ the Euler–Poincaré function at infinitesimal character $\xi $ . If $\sigma $ is unitary, this is only possible if $\sigma $ has nonzero $(\mathfrak {g},K)$ -cohomology with respect to the irreducible finite-dimensional representation of infinitesimal character $\xi $ .

By the main classification result of [Reference Vogan and ZuckermanVZ84], the only representations that do so are the discrete-series packet $\Pi _\lambda (\xi - \rho _G)$ and certain cohomological inductions $A_{\mathfrak {q}}(\lambda )$ for $\theta $ -stable parabolic subalgebras $\mathfrak {q}$ of $\mathfrak {g}$ and $\lambda $ a character of the Levi algebra $\mathfrak {l}$ associated to $\mathfrak {q}$ (see, for example, [Reference Adams and JohnsonAJ87, §2.1] for a definition of $A_{\mathfrak {q}}(\lambda )$ ). It therefore suffices to show that none of these except $\pi $ itself have $\pi $ appearing in their character formulas.

The only nontrivial case to check is that of nondiscrete-series $A_{\mathfrak {q}}(\lambda )$ . Theorem 8.2 in [Reference Adams and JohnsonAJ87] provides its character formula and shows that the discrete series that appear are exactly those with Harish–Chandra parameters of the form $\lambda + \omega \rho _{\mathfrak {l}}$ , where $\omega $ ranges over the Weyl group of $\mathfrak {l}$ . For each $\omega $ , pick a set of simple roots of $\mathfrak {l}$ so that $\omega \rho _l$ is dominant. Then for simple root $\alpha $ of $\mathfrak {l}$ ,

$$\begin{align*}\langle \lambda + \omega \rho_{\mathfrak{l}}, \alpha \rangle = \langle \omega \rho_{\mathfrak{l}}, \alpha \rangle = 1. \end{align*}$$

In particular, if $\pi $ appears in the character formula for $A_{\mathfrak {q}}(\lambda )$ , then there is a choice of simple roots of $\mathfrak {l}$ that are in $S_\xi $ .

Finally, since $\lambda $ is regular, for any root $\alpha $ of G, $|\langle \lambda , \alpha ^\vee \rangle | \geq 1$ . Therefore, $S_\xi $ needs to be a subset our simple roots chosen to make $\lambda G$ -dominant. Let $\mathfrak {l}_\xi $ be the associated Levi subalgebra. If $\pi $ appears in the character formula for $A_{\mathfrak {q}}(\lambda )$ , the above gives that $\mathfrak {l} \subseteq \mathfrak {l}_\xi $ . Therefore, if $S_\xi $ has no noncompact roots, then $\mathfrak {l}$ is compact, so our condition on $\mathfrak {l}$ implies that $A_{\mathfrak {q}}(\lambda )$ is discrete series (see, for example, the bottom of [Reference Adams and JohnsonAJ87, pg. 272]) and therefore equal to $\pi $ . In total, $\pi $ cannot appear in other character formulas completing one direction.

In the other direction, if $S_\xi $ has a noncompact root, then this can be used to construct a rank- $1$ Levi subalgebra $\mathfrak {l}$ that isn’t compact. Pick corresponding $\mathfrak {q}$ , and choose chamber for $\mathfrak {l}$ so that $\lambda $ is $\mathfrak {l}$ -dominant. Then $\pi $ will appear in the character formula of $A_{\mathfrak {q}}(\lambda - \rho _{\mathfrak {l}})$ which isn’t discrete series.

2.3 Quaternionic discrete series

Quaternionic discrete series are a special class of discrete series picked out in [Reference Gross and WallachGW96]. We recall some needed definitions and properties:

By looking at extended root diagrams:

Then, by Blattner’s formula for minimal K-types:

Miraculously, almost all quaternionic discrete series are trace distinguishable:

3.1 Spectral side

The previous discussion on quaternionic discrete series shows:

Theorem 3.1.1. Let $G_\infty $ have quaternionic discrete series, and let $\pi _0$ be a quaternionic discrete series of $G_\infty $ with infinitesimal character not equal to $\rho _G$ . Then

$$ \begin{align*} I_{\mathrm{spec}}(\varphi_{\pi_0} \otimes f^\infty) & = \sum_{\pi \in \mathcal{AR}_{\mathrm{disc}}(G_2)} m_{\mathrm{disc}}(\pi) \delta_{\pi_\infty = \pi_0} \operatorname{\mathrm{tr}}_{\pi^\infty} (f^\infty) \\ &= \sum_{\pi \in \mathcal{AR}_{\mathrm{cusp}}(G_2)} m_{\mathrm{cusp}}(\pi) \delta_{\pi_\infty = \pi_0} \operatorname{\mathrm{tr}}_{\pi^\infty} (f^\infty). \end{align*} $$

3.2 Geometric side/the hyperendoscopy formula

3.2.2 Preliminaries

Let $\pi _0$ be a quaternionic discrete series on $G_\infty $ . We will use the hyperendoscopy formula of [Reference FerrariFer07] to compute $I_{\mathrm {geom}}(\varphi _{\pi _0} \otimes f^\infty )$ . We need to apply the general case of [Reference DalalDal22, Thm. 4.2.3] since G might have endoscopy without simply connected derived subgroup. We use notation from [Reference DalalDal22, §3,4] to discuss endoscopy and hyperendoscopy. See [Reference Kottwitz and ShelstadKS99] for a full reference to the theory of endoscopy and [Reference LabesseLab11] for a course-notes-style introduction.

Let $\eta $ be the Euler–Poincaré function for the L-packet $\Pi _{\mathrm {disc}}(\lambda )$ that conatins $\pi _0$ . Let $\mathcal {HE}_{\mathrm {ell}}(G)$ be the set of nontrivial hyperendoscopic paths for G. Then, in the notation of [Reference DalalDal22, §4],

$$\begin{align*}I^{G_2}_{\mathrm{geom}}(\varphi_{\pi_0} \otimes f^\infty) = I^{G_2}_{\mathrm{geom}}(\eta_k \otimes f^\infty) + \sum_{H \in \mathcal{HE}_{\mathrm{ell}}(G_2)} \iota(G, \mathcal{H}) I^{\tilde{\mathcal{H}}}_{\mathrm{geom}}(((\eta - \varphi_{\pi_0}) \otimes f^\infty)^{\tilde{\mathcal{H}}}), \end{align*}$$

where the $\tilde {\mathcal {H}}$ are choices of z-pair paths when they are needed.

3.2.3 Telescoping

Next, an unpublished result of Kottwitz summarized in [Reference MorelMor10, §5.4] and proved by other methods in [Reference PengPen19] stabilizes $I_{\mathrm {geom}}(\varphi \otimes f^\infty )$ when $\varphi $ satisfies a technical property of being stable-cuspidal (as EP-functions are but pseudocoefficients are not):

Theorem 3.2.1. Let $\varphi $ be stable cuspidal (e.g., an EP-function, but not a pseudocoefficient) on $G_\infty $ and $f^\infty $ a test function on $G(\mathbb A^\infty )$ . Then

$$\begin{align*}I^{G}_{\mathrm{geom}}(\varphi \otimes f^\infty) = \sum_{H \in \mathcal{E}_{\mathrm{ell}}(G)} \iota(G,H) S^{\tilde{H}}_{\mathrm{geom}}((\varphi \otimes f^\infty)^{\tilde{H}}), \end{align*}$$

where $\mathcal {E}_{\mathrm {ell}}(G)$ is the set of elliptic endoscopic groups for G and the $\tilde {H}$ are z-extensions if necessary. The transfers $(\varphi \otimes f^\infty )^{\tilde {H}}$ depend on choices of measures for G and H.

The $S_{\mathrm {geom}}$ terms are defined by their values on Euler–Poincaré functions:

$$ \begin{align*} S^H_{\mathrm{geom}}(\eta_\lambda \otimes f^\infty) &= \sum_{M \in \mathcal{L}^{\mathrm{cusp}}(H)} (-1)^{[H : M]} \frac{|\Omega_{M,F}|}{|\Omega_{H,F}|} \tau(M) \\ &\quad \times \sum_{\gamma \in [M(\mathbb Q)]_{{\mathrm{st}}, \mathrm{ell}^\infty}} |\iota^M(\gamma)|^{-1} \frac{e(\bar M_{\gamma, \infty})}{ \operatorname{\mathrm{vol}}(\bar M_{\gamma, \infty}/A_{\bar M_\gamma, \infty})} \frac{k(M_\infty)}{k(H_\infty)} \Phi^H_M(\gamma, \lambda) SO_\gamma^\infty((f^\infty)_M), \end{align*} $$

choosing Tamagawa globally measure on all centralizers. The volume on $\bar M_{\gamma , \infty }$ is transferred from that on $M_{\gamma , \infty }$ in the standard way for inner forms so that the entire term doesn’t depend on a choice of measure at infinity.

There’s an alternating sign in the hyperendoscopy formula: If $\mathcal {H}$ is a hyperendoscopic path, then $-\iota (G, \mathcal {H}) \iota (\mathcal {H}, H) = \iota (G, (\mathcal {H}, H))$ for H any endoscopic group of $\mathcal {H}$ . Here, $(\mathcal {H}, H)$ represents the concatenation and $\mathcal {H}$ is overloaded to also refer to the last group in $\mathcal {H}$ .

In particular, substituting in the stabilization telescopes the hyperendoscopy formula.

3.3 Final formula and usage notes

3.3.1 Formula

Telescoping together with Theorem 3.1.1 produces the final formula for quaternionic discrete series $\pi _0$ of $G_\infty $ that has infinitesimal character not equal to $\rho _G$ :

(1) $$ \begin{align} \sum_{\pi \in \mathcal{AR}_{\mathrm{disc}}(G)}& m_{\mathrm{disc}}(\pi) \delta_{\pi_\infty = \pi_0} \operatorname{\mathrm{tr}}_{\pi^\infty} (f^\infty) \nonumber\\ &\quad= S^{G}_{\mathrm{geom}}(\eta \otimes f^\infty) + \sum_{\substack{H \in \mathcal{E}_{\mathrm{ell}}(G) \\ H \neq G}} \iota(G, H) S^{\tilde{H}}_{\mathrm{geom}}((\varphi_{\pi_0} \otimes f^\infty)^{\tilde{H}}). \end{align} $$

The right side can be evaluated with Theorem 3.2.1.

We recall

$$\begin{align*}\iota(G,H) = |\Lambda(H, \mathcal{H}, s, \eta)|^{-1} \frac{\tau(G)}{\tau(H)}, \end{align*}$$

where $\Lambda (H, \mathcal {H}, s, \eta )$ is the image in $\operatorname {\mathrm {Out}}(\widehat H)$ of the automorphisms of the endoscopic quadruple.

While getting a formula in terms of the distributions $S_{\mathrm {geom}}$ on smaller endoscopic groups comes immediately from stabilization, the above telescoping argument seems to be necessary to get explicit formulas for the $S_{\mathrm {geom}}$ when using a test factor at infinity that is just cuspidal instead of a stable cuspidal.

3.3.2 Usage

There are two possible methods to compute terms here. If we were interested in working with more general groups or at more general level, something like method 1 would have been necessary. However, our application case of level- $1$ representations on $G_2$ allows us to use the much easier method 2. Method 2 in fact does not even need an explicit expansion for $S_{\mathrm {geom}}$ .

Method 1:

We can try calculate the $S_{\mathrm {geom}}$ terms directly from their formula in Theorem 3.2.1. We will need to choose Euler–Poincaré measure at $\bar M_\gamma $ times canonical measure for the orbital integrals (canonical measure is the same for all inner forms). This adds an extra factor of

$$\begin{align*}d(\bar M_{\gamma, \infty}) \frac{L(\mathrm{Mot}_{M_{\gamma}})}{e(\bar M_{\gamma, \infty}) 2^{\operatorname{\mathrm{rank}}(M_{\gamma, \infty})}} \end{align*}$$

by [Reference Shin and TemplierST16, lem. 6.2]. Since $d(H_\infty ) = 1$ and $\operatorname {\mathrm {vol}}_{EP}(H_\infty /A_{H_\infty }) = 1$ for H compact, this expands the terms in Equation (1) as

$$ \begin{align*} &S^H_{\mathrm{geom}}(\eta_\lambda \otimes f^\infty) = \sum_{M \in \mathcal{L}^{\mathrm{cusp}}(H)} \left( (-1)^{[H : M]} \frac{|\Omega_{M,F}|}{|\Omega_{H,F}|} \right) \left( \tau(M) \frac{k(M_\infty)}{k(H_\infty)} \right) \\ &\qquad\times \sum_{\gamma \in [M(\mathbb Q)]_{{\mathrm{st}}, \mathrm{ell}^\infty}} 2^{-\operatorname{\mathrm{rank}}(M_{\gamma, \infty})} \Phi^H_M(\gamma, \lambda) \left( L(\mathrm{Mot}_{M_{\gamma}}) |\iota^M(\gamma)|^{-1} SO_\gamma^\infty((f^\infty)_M) \right), \end{align*} $$

where the stable orbital integrals are now computed using canonical measure on centralizers.

The hardest terms here are the stable orbital integrals, the L-values and the characters $\Phi $ . The constant terms $(f^\infty )_M$ are explicit integrals.

The L-values may be computed as products of values of Artin L-functions by explicitly describing the motives from [Reference GrossGro97]. The terms $\Phi $ can be reduced to linear combinations of traces of $\gamma $ against finite-dimensional representations of G by the algorithm on [Reference ArthurArt89, pg. 273]. These can be computed by the Weyl character formula and its extension to irregular elements stated in, for example, [Reference Chenevier and RenardCR15, prop. 2.3].

The stable orbital integrals unfortunately cause far more difficulty. For specific groups, including our eventual application case of $G_2$ , they are computed and listed in tables in [Reference Gross and PollackGP05, pg. 159]. First, they are interpreted as orbital integrals on compact-at-infinity form $G^c$ by endoscopic transfer. The spectral side of the trace formula on $G^c$ is then possible to compute, allowing the orbital integrals to be solved for once the coefficients in terms of L-values are known. Alternatively, [Reference Chenevier and TaïbiCT20] uses another trick, inputting vanishing results for small weight automorphic representations to solve for unstable orbital integrals in the resulting system of linear equations.

They can also be computed directly from unstable orbital integrals: [Reference KottwitzKot77] and [Reference TaïbiTaï17] use Bruhat–Tits theory to do this for $\mathrm {GL}_3$ and some classical groups respectively. Either way, all currently known methods are not fully general and extremely complicated.

Method 2:

Fortunately, there is a much simpler way to compute for our desired application of level- $1$ representations on $G_2$ . Recalling that $I^{G_2^c}$ is known from [Reference Chenevier and RenardCR15], we can compare the expansions (1) for $G_2$ and $G_2^c$ . The term for $S^{G_2}$ a appears in the expansion for $I^{G_2^c}$ and can therefore be solved for and substituted in the expansion for $I^{G_2}$ . In total, we get a formula

$$\begin{align*}I^{G_2} = I^{G_2^c} + \text{corrections}, \end{align*}$$

where the corrections are in terms of $S^H$ for smaller endoscopic H.

In the next section, we will see that there aren’t actually that many H appearing. Finally, Section 7 will show that the terms for these H are easily computed through another trick in the case of level- $1$ . Method 2 also gives in Section 8 a Jacquet–Langlands-style result comparing quaternionic representations on $G_2$ to representations on $G_2^c$ .

Note. We comment on possible extensions of method 2. The comparison to a compact form would work for any group with a form that is compact at infinity and unramified at all finite places. These appear in types, $G_2, B_3, D_4, B_4, F_4, B_7, D_8, B_8$ and $E_8$ as enumerated in [Reference GrossGro96].

Being able to easily count the endoscopic terms spectrally is more rare and requires some kind of recursive expansion down to only terms of Lie type $A_1^n$ . This in particular works out for type $D_4$ , so level- $1$ forms on type $D_4$ should be countable analogously to method 2.

In another direction, plugging in other unramified test functions could compute counts weighted by Hecke eigenvalues. These would be in terms of the same weighted counts on $G_2^c$ and certain other weighted counts of classical modular forms that are determined by combinatorial formulas for unramified transfers as explained in [Reference DalalDal22, §5.4].

4.1 Root system of $G_2$

4.1.1 Roots

We use notation from [Reference Li and SchwermerLS93] to specify the root system of $G_2$ . Let K be the maximal compact $\mathrm {SU}(2) \times \mathrm {SU}(2)/\pm 1$ of $G_2(\mathbb R)$ , and choose a maximal torus $T(\mathbb R)$ that is inside K. Make a choice of simple roots of $G_2(\mathbb R)$ that are noncompact; in this case determining a unique dominant chamber with respect to both $G_2$ and K. Let $\beta $ be the highest root of $G_2$ with respect to the choice of simple roots and note that it is long.

We now give explicit coordinates. As a mnemonic convention, roots indexed $1$ will be short and roots indexed $2$ will be long. Figure 1 displays all the roots and shades our choices of dominant Weyl chambers for both $G_2$ and K. The compact roots at infinity are the four along the $\epsilon _i$ -coordinate axes.

Figure 1 Character lattice, roots and choices of dominant chamber for $G_2$ .

If the roots of the short and long $\mathrm {SU}_2$ are $2\epsilon _1$ and $2\epsilon _2$ respectively, then the simple roots of $G_2$ are

$$\begin{align*}\text{(short) }\alpha_1 = - \epsilon_1 + \epsilon_2, \qquad \text{(long) } \alpha_2 = 3 \epsilon_1 - \epsilon_2. \end{align*}$$

The other positive roots are

$$ \begin{gather*} \text{(short) } 2 \epsilon_1 = \alpha_1 + \alpha_2, \qquad \epsilon_1 + \epsilon_2 = 2 \alpha_1 + \alpha_2, \\ \text{(long) } 2 \epsilon_2 = 3 \alpha_1 + \alpha_2, \qquad 3 \epsilon_1 + \epsilon_2 = 3 \alpha_2 + 2 \alpha_2. \end{gather*} $$

The fundamental weights are

$$\begin{align*}\lambda_1 = 2 \alpha_1 + \alpha_2, \qquad \lambda_2 = 3 \alpha_1 + 2 \alpha_2. \end{align*}$$

Of course $\beta = \lambda _2$ .

The Weyl group is generated by simple reflections

$$\begin{align*}s_{\alpha_1} \begin{pmatrix} 2\epsilon_1 \\ 2\epsilon_2 \end{pmatrix} = \begin{pmatrix} \epsilon_1 + \epsilon_2 \\ 3\epsilon_1 - \epsilon_2 \end{pmatrix}, \qquad s_{\alpha_2} \begin{pmatrix} 2\epsilon_1 \\ 2\epsilon_2 \end{pmatrix} = \begin{pmatrix} -\epsilon_1 + \epsilon_2 \\ 3\epsilon_1 + \epsilon_2 \end{pmatrix}. \end{align*}$$

Finally,

$$ \begin{gather*} \rho_K = \epsilon_1 + \epsilon_2 = 2 \alpha_1 + \alpha_2, \\ \rho_G = 4 \epsilon_1 + 2 \epsilon_2 = 5 \alpha_1 + 3 \alpha_2. \end{gather*} $$

4.1.2 Coroots

Coroots will follow the opposite mnemonic: Coroots indexed $1$ are long, and coroots indexed $2$ are short.

Since $G_2$ has trivial center, $X^*(T)$ is the root lattice, which is exactly

$$\begin{align*}X^*(T) = \{a \epsilon_1 + b \epsilon_2 : a,b \in \mathbb Z, a + b \in 2\mathbb Z\}. \end{align*}$$

Let $(\delta _1, \delta _2)$ be the dual basis to $(2\epsilon _1, 2\epsilon _2)$ : that is, $(\delta _i, \epsilon _j) = 1/2 {\mathbf {1}}_{i=j}$ . Then

$$\begin{align*}X_*(T) = \{a \delta_1 + b \delta_2 : a,b \in \mathbb Z, a + b \in 2\mathbb Z\}. \end{align*}$$

Since $\epsilon _1$ and $\epsilon _2$ are perpendicular,

$$ \begin{gather*} (2 \epsilon_1)^\vee = 2\delta_1, \\ (2 \epsilon_2)^\vee = 2\delta_2. \end{gather*} $$

More generally, the Weyl action gives

$$ \begin{gather*} (\alpha_1^\vee, 2 \epsilon_1) = -1, \qquad (\alpha_1^\vee, 2\epsilon_2) = 3,\\ (\alpha_2^\vee, 2 \epsilon_1) = 1, \qquad (\alpha_2^\vee, 2\epsilon_2) = - 1, \end{gather*} $$

so we get simple coroots

$$ \begin{gather*} \alpha_1^\vee = -\delta_1 + 3\delta_2, \\ \alpha_2^\vee = \delta_1 - \delta_2. \end{gather*} $$

This reproduces that the coroot lattice is $X_*(T)$ , implying that $G_2$ is simply connected. For completeness,

$$ \begin{gather*} \lambda_1^\vee = \delta_1 + 3 \delta_2,\\ \lambda_2^\vee = \delta_1 + \delta_2. \end{gather*} $$

4.2 Quaternionic discrete series for $G_2$

The quaternionic discrete series on $G_2$ of weight k for $k \geq 2$ lies in the L-packet

$$\begin{align*}\Pi^{G_2}_{\mathrm{disc}}((k-2) \beta). \end{align*}$$

The members of this L-packet have Harish–Chandra parameters:

$$\begin{align*}(k-2)\beta + \rho_G, \qquad s_{\alpha_1}((k-2)\beta + \rho_G), \qquad s_{\alpha_2}((k-2)\beta + \rho_G). \end{align*}$$

As in [Reference Gan, Gross and SavinGGS02], the quaternionic member is the one with minimal K-type $\lambda _B = 2k\epsilon _2$ . We know that the discrete series $\pi (\omega , \lambda )$ has minimal K-type

$$\begin{align*}\lambda_B = \omega(\lambda + 2 \rho_G) - 2 \rho_K \end{align*}$$

by the Blattner formula [Reference KnappKna01, Thm. 9.20]. Therefore, the weight-k quaternionic discrete series $\pi _k$ is specifically $\pi (s_{\alpha _2}, (k-2)\beta )$ – computing, $s_{\alpha _2}$ fixes $\rho _K$ so

$$\begin{align*}s_{\alpha_2}(\lambda + 2 \rho_G) - 2 \rho_K = s_{\alpha_2}(\lambda + 2 \rho_G - 2 \rho_K) = s_{\alpha_2}(\lambda + 2 \beta) = s_{\alpha_2}(k \beta) = 2k \epsilon_2. \end{align*}$$

This is the discrete series with Harish–Chandra parameter

$$\begin{align*}\lambda_{k,H}: = s_{\alpha_2}((k-2)\beta + \rho_G). \end{align*}$$

Call it $\pi _k$ and its pseudocoefficient $\varphi _k$ .

Theorem 3.1.1 then gives that for $k> 2$ ,

(2) $$ \begin{align} |\mathcal{Q}_k(1)| = I_{\mathrm{spec}}(\varphi_k \otimes {\mathbf{1}}_K) \end{align} $$

if we choose measures so that $\operatorname {\mathrm {vol}} G_2(\widehat {\mathbb {Z}}) = 1$ . Note again that this heavily depends on the miracle of Proposition 2.3.3 and a similar result does not hold either for pseudocofficients for the other members of $\Pi _{\mathrm {disc}}((k-2)\beta )$ or for the Euler–Poincaré function.

5.1 Elliptic endoscopy of $G_2$

The elliptic endoscopic groups of $G_2$ are $G_2$ , $\mathrm {PGL}_3$ and $\mathrm {SO}_4$ . This is stated in a thesis [Reference AltschulAlt13] but not fully explained, so we fill in some details here for reader convenience. We use notational conventions for endoscopy as in [Reference DalalDal22, §3]. We compute the possible endoscopic pairs $(s, \rho )$ .

Since $G_2$ has trivial center, the cohomology condition on s is always satisfied so we don’t bother checking it. Trivial center further gives that the isomorphism class of the pair only depends on s through its centralizer. Next, we note a result that we thank a referee for pointing out:

In particular, $\rho $ is always trivial and we can always find a valid s for any possible centralizer $\widehat H$ . The possible elliptic $\widehat H$ with trivial $\rho $ are $G_2, \mathrm {SL}_3$ and $\mathrm {SL}_2 \times \mathrm {SL}_2/\{\pm 1\}$ corresponding to split endoscopic groups $G_2, \mathrm {PGL}_3$ , and $\mathrm {SL}_2 \times \mathrm {SL}_2/\{\pm 1\}$ . In each of these cases, $\Lambda = 1$ .

If a group contributes to the stabilization applied to our test function, then by the fundamental lemma, it needs to be unramified away from infinity. By formulas for transfers of pseudocoefficients (see [Reference DalalDal22, lem. 5.6.1]), it also needs to have an elliptic maximal torus at infinity. The only groups contributing are therefore the $G_2$ and the $\mathrm {SL}_2 \times \mathrm {SL}_2/\{\pm 1\}$ .

5.2 Endoscopic constants and normalizations

5.2.1 The $\iota $

Let $H = \mathrm {SL}_2 \times \mathrm {SL}_2/\pm 1$ , and let $G_2^c$ be the unique nonsplit inner form of $G_2$ over $\mathbb Q$ which is compact at infinity. Then,

$$ \begin{gather*} \iota(G_2^c, H) = \iota(G_2, H) = |\Lambda(H, \mathcal{H}, s, \eta)|^{-1} \frac{\tau(G)}{\tau(H)} = 1 \cdot \frac12, \\ \iota(G_2^c, G_2) = 1, \end{gather*} $$

by Kottwitz’s formula for Tamagawa numbers (note that $\ker ^1(\mathbb Q, Z_H) = \ker ^1(\mathbb Q, \{\pm 1\}) =1$ ).

5.2.3 The stabilizations

We fix canonical measure at finite places so that the fundamental lemma directly gives ${\mathbf {1}}_{K^\infty _{G_2}}^H = {\mathbf {1}}_{K^\infty _H}$ . Recall that EP-functions and pseudocoefficients are defined depending on measure so we don’t need to fix measure at infinity.

Then, Equation (1) gives

(3) $$ \begin{align} I^{G_2}(\varphi_{\pi_{G_2}(s_{\alpha_2}, (k-2) \beta)} \otimes &{\mathbf{1}}_{K^\infty_{G_2}}) \nonumber\\&= S^{G_2}(\eta^{G_2}_{(k-2)\beta} \otimes {\mathbf{1}}_{K^\infty_{G_2}}) + \frac12 S^H((\varphi_{\pi_{G_2}(s_{\alpha_2}, (k-2) \beta)})^H \otimes {\mathbf{1}}_{K^\infty_{H}}). \end{align} $$

A simple case of the discrete transfer formula in [Reference LabesseLab11, §IV.3] computes that $(\eta ^{G_2^c}_{(k-2)\beta })^{G_2} = \eta ^{G_2}_{(k-2)\beta }$ (note that $\Omega _{\mathbb R}(G_2^c) \backslash \Omega _{\mathbb C}(G_2^c)$ is trivial so $\kappa $ is too), so

$$\begin{align*}I^{G_2^c}(\eta^{G_2^c}_{(k-2)\beta} \otimes {\mathbf{1}}_{K^\infty_{G_2^c}}) = S^{G_2}(\eta^{G_2}_{(k-2)\beta} \otimes {\mathbf{1}}_{K^\infty_{G_2}}) + \frac12 S^{H}((\eta^{G^c_2}_{(k-2)\beta})^H \otimes {\mathbf{1}}_{K^\infty_{H}}). \end{align*}$$

Since type $A_1 \times A_1$ has no nontrivial centralizer of full semisimple rank, all elliptic endoscopy of $\mathrm {SL}_2 \times \mathrm {SL}_2/\pm 1$ is nonsplit. Therefore, it is ramified at some prime, so the transfers of ${\mathbf {1}}_{K^\infty _H}$ vanish, implying that $S^H = I^H$ on our test functions. Substituting one stabilization into another finally gives

(4) $$ \begin{align} I^{G_2}(\varphi_{\pi_{G_2}(s_{\alpha_2}, (k-2) \beta)} &\otimes {\mathbf{1}}_{K^\infty_{G_2}}) = I^{G_2^c}(\eta^{G_2^c}_{(k-2)\beta} \otimes {\mathbf{1}}_{K^\infty_{G_2^c}}) \nonumber\\&- \frac12 I^H ((\eta^{G^c_2}_{(k-2)\beta})^H \otimes {\mathbf{1}}_{K^\infty_{H}}) + \frac12 I^H((\varphi_{\pi_{G_2}(s_{\alpha_2}, (k-2) \beta)})^H \otimes {\mathbf{1}}_{K^\infty_{H}}) \end{align} $$

under canonical measure at finite places.

This is our realization of method 2. There are three steps remaining to get counts:

1. 1. Compute the transfers of EP-functions to H.

2. 2. Write the resulting $I^H(\eta _\lambda \otimes {\mathbf {1}}_{K_H})$ terms in terms of counts of level- $1$ , classical modular forms.

3. 3. Look up values for the $G_2^c$ -term from [Reference Chenevier and RenardCR15].

6.1 Root combinatorics

Since $\rho _G - \rho _H \in X^*(T)$ , [Reference LabesseLab11, §IV.3] gives the transfer of the pseudocoefficient of the quaternionic discrete series to H:

(5) $$ \begin{align} &(\varphi_{\pi_{G_2}(s_{\alpha_2}, (k-2) \beta)})^H= \nonumber\\&\quad \kappa^H(s_{\alpha_2}^{-1}) \eta^H_{(k-2)\beta + \rho_G - \rho_H} - \kappa^H(s_{\alpha_1} s_{\alpha_2}^{-1}) \eta^H_{s_{\alpha_1}((k-2)\beta + \rho_G) - \rho_H} - \eta^H_{s_{\alpha_2}((k-2)\beta + \rho_G) - \rho_H} \end{align} $$

for some signs $\kappa ^H(\cdot )$ .

We compute that $\rho _H = \epsilon _1 + \epsilon _2$ . Then

$$\begin{align*}(k-2)\beta + \rho_G - \rho_H = (k-2)(3 \epsilon_1 + \epsilon_2) + (3 \epsilon_1 + \epsilon) = 3(k-1) \epsilon_1 + (k-1) \epsilon_2. \end{align*}$$

In addition,

$$ \begin{align*} s_{\alpha_1} \rho_G = 5\epsilon_1 + \epsilon_2, &\qquad s_{\alpha_1} \beta = \beta, \\ s_{\alpha_2} \rho_G = \epsilon_1 + 3\epsilon_2, &\qquad s_{\alpha_2} \beta = 2 \epsilon_2, \end{align*} $$

so

$$\begin{align*}s_{\alpha_1}((k-2)\beta + \rho_G) - \rho_H = (k-2)(3 \epsilon_1 + \epsilon_2) + (4 \epsilon_1) = (3k-2) \epsilon_1 + (k-2) \epsilon_2 \end{align*}$$

and

$$\begin{align*}s_{\alpha_2}((k-2)\beta + \rho_G) - \rho_H = (k-2)(2\epsilon_2) + (2 \epsilon_2) = 2(k-1)\epsilon_2. \end{align*}$$

6.2 Endoscopic characters

6.2.1 Setup

It remains to compute the $\kappa $ terms in Equation (5). These signs depend in a very complicated way on the realization of H and the exact transfer factors chosen. We will therefore use an indirect trick to compute them more easily.

Let $\psi _H$ be a (discrete in our case) L-parameter for $H(\mathbb R)$ and $\psi _G$ the composition with ${}^L\!{H} \hookrightarrow {}^L\!{G}_2$ . Then we have an identity of traces over L-packets:

$$\begin{align*}S \Theta_{\psi_H}(f^H) = \sum_{\pi \in \Pi_{\psi_G}} \langle s_H, \pi \rangle \Theta_\pi(f), \end{align*}$$

where $f^H$ is a transfer of f, $\Theta _\pi $ is the Harish–Chandra character, $S\Theta _{\psi _H}$ is the stable character corresponding to the L-packet, $\Pi _{\psi _G}$ is the L-packet corresponding to the L-parameter, and $\langle s_H, \pi \rangle $ is shorthand for a sign depending on $\pi $ and the choice of Whittaker data. This sign comes from pairing an element $s_H$ of the centralizer of $\psi _G$ determined by H with a character associated to $\pi $ through the Whittaker datum. See [Reference KalethaKal16, §1] for an exposition of how this works in general.

If $\pi $ on $G_2$ is discrete series, Labesse’s formula tells us that we can choose

$$\begin{align*}(\varphi^{G_2}_\pi)^H = \sum_\lambda \epsilon(\lambda, \pi) \eta^H_\lambda \end{align*}$$

for some signs $\epsilon $ that depend on the transfer factor and some set of weights $\lambda $ that only depends on the infinitesimal character of $\pi $ .

Let $\psi _\lambda $ be the L-parameter corresponding to weight- $\mu $ discrete series on H. Plugging this formula for $\varphi ^{G_2}_\pi $ into the trace identity for $\psi _\lambda $ gives that

$$\begin{align*}\epsilon(\lambda, \pi) = \sum_\mu \epsilon(\mu, \pi) S \Theta_{\psi_\lambda}(\eta_\mu^H) = \sum_{\pi' \in \Pi_{\psi_G}} \langle s_H, \pi' \rangle \Theta_{\pi'}(\varphi^{G_2}_\pi) = \begin{cases} \langle s_H, \pi \rangle & \pi \in \Pi_{\psi_G} \\ 0 & \text{else} \end{cases}, \end{align*}$$

where $\psi _G$ is the pushforward of $\psi _\lambda $ . The last equality is the definition of pseudocoefficient since all $\pi $ in a packet for an L-parameter should be tempered. This computation shows that $\psi _\lambda $ is required to push forward to the parameter for $\pi $ and that $\epsilon (\lambda , \pi ) = \langle s_H, \pi \rangle $ .

6.2.2 The trick

Now, we are ready to compute the signs. Instead of doing the hard work of figuring out how the transfer factor directly affects the signs in Labesse’s formulation, we will use the key fact that $\epsilon (\lambda , \pi ) = \langle s_H, \pi \rangle = 1$ whenever $\pi $ is the Whittaker-generic member of its L-packet. Therefore, in Labesse’s formula for the generic member $\pi _{1, (k-2)\beta }$ ,

$$ \begin{align*} &(\varphi_{\pi_{G_2}(1, (k-2) \beta)})^H = \eta^H_{(k-2)\beta + \rho_G - \rho_H} + \kappa^H(s_{\alpha_1}) \operatorname{\mathrm{sgn}}(s_{\alpha_1}) \eta^H_{s_{\alpha_1}((k-2)\beta + \rho_G) - \rho_H}\\ &+ \kappa^H(s_{\alpha_2}) \operatorname{\mathrm{sgn}}(s_{\alpha_2}) \eta^H_{s_{\alpha_2}((k-2)\beta + \rho_G) - \rho_H}, \end{align*} $$

all the coefficients need to be $1$ . The allows to solve

$$\begin{align*}\kappa^H(s_{\alpha_1}) = \kappa^H(s_{\alpha_2}) = -1 \end{align*}$$

for our choice of transfer factors. Right- $\Omega _{\mathbb R}$ -invariance of Labesse’s $\kappa $ then also gives that

$$\begin{align*}\kappa^H(s_{\alpha_1}s_{\alpha_2}) = -1. \end{align*}$$

6.3 Final formulas for transfers

Therefore, our final transfer is

(6) $$ \begin{align} (\varphi_{\pi_{G_2}(s_{\alpha_2}, (k-2) \beta)})^H = -\eta^H_{3(k-1) \epsilon_1 + (k-1) \epsilon_2} + \eta^H_{(3k-2) \epsilon_1 + (k-2) \epsilon_2} - \eta^H_{2(k-1)\epsilon_2}. \end{align} $$

Transfers from $G_2^c$ are easier. Here, $\Omega _{\mathbb R}(G_2^c) \backslash \Omega _{\mathbb C}(G_2^c)$ is trivial so the average value of $\kappa $ is $1$ . Averaging Labesse’s formula as in [Reference DalalDal22, cor. 5.1.5] therefore gives

(7) $$ \begin{align} (\eta^{G_2^c}_{(k-2) \beta})^H = \eta^H_{3(k-1) \epsilon_1 + (k-1) \epsilon_2} - \eta^H_{(3k-2) \epsilon_1 + (k-2) \epsilon_2} - \eta^H_{2(k-1)\epsilon_2}. \end{align} $$

7.1 Cohomological representations of $H(\mathbb R)$

Next, we recall that the infinite trace measures an Euler characteristic against $(\mathfrak {h}, K_{H,\infty })$ -cohomology:

$$\begin{align*}\operatorname{\mathrm{tr}}_{\pi_\infty}(\eta^H_\lambda) = \chi(H^*(\mathfrak{h}, K_{H,\infty}, \pi_\infty \otimes V_\lambda)), \end{align*}$$

where $\mathfrak {h}$ is the Lie algebra of $H_\infty $ and $V_\lambda $ is the finite-dimensional representation of weight $\lambda $ of $H^0_\infty $ pulled back to $H_\infty $ . Using the definition from [Reference Borel and WallachBW00, §5.1],

$$\begin{align*}H^*(\mathfrak{h}, K_{H,\infty}, \pi_\infty \otimes V_\lambda) = H^*(\mathfrak{h}, K^0_{H,\infty}, \pi_\infty \otimes V_\lambda)^{K_{H,\infty}/K^0_{H,\infty}}, \end{align*}$$

it suffices to consider the $\pi _\infty $ whose restrictions to $H_\infty ^0$ contain a component that is cohomological when pulled back to $[\mathrm {SL}_2 \times \mathrm {SL}_2](\mathbb R)$ . By Frobenius reciprocity and semisimplicity of inductions, these are exactly the irreducible constituents of $\operatorname {\mathrm {Ind}}_{H_\infty ^0}^{H_\infty } \pi '$ for $\pi '$ cohomological of $H_\infty ^0$ .

Next, $H_\infty ^0$ is index $2$ in $H_\infty $ . Pick $h \in H_\infty - H_\infty ^0$ , and let $\pi ^{\prime (h)}$ be the representation $\gamma \mapsto \pi '(h^{-1} \gamma h)$ . Define character

$$\begin{align*}\chi : H_\infty \mapsto H_\infty/H_\infty^0 \simeq \{\pm 1\}. \end{align*}$$

As noted in the proof of Lemma 2.5 in [Reference Labesse and LanglandsLL79], there are two cases for $H_\infty ^0$ -representations $\pi '$ :

1. 1. $\pi ' \neq \pi ^{\prime (h)}$ : Then $\operatorname {\mathrm {Ind}}_{H_\infty ^0}^{H_\infty } \pi '$ is irreducible and $\operatorname {\mathrm {Res}}_{H_\infty ^0}^{H_\infty }\operatorname {\mathrm {Ind}}_{H_\infty ^0}^{H_\infty } \pi ' = \pi ' \oplus \pi ^{\prime (h)}$ .

2. 2. $\pi ' = \pi ^{\prime (h)}$ : Then $\operatorname {\mathrm {Ind}}_{H_\infty ^0}^{H_\infty } \pi ' = V \oplus (V \otimes \chi )$ for some irreducible V. Also, $\operatorname {\mathrm {Res}}_{H_\infty ^0}^{H_\infty }\operatorname {\mathrm {Ind}}_{H_\infty ^0}^{H_\infty } \pi ' = \pi ' \oplus \pi '$

Recalling a standard result, the cohomological representations of $\mathrm {SL}_2(\mathbb R)$ with respect to $\lambda $ are:

• • A discrete series L-packet $\{\pi _{\lambda ,1}$ , $\pi _{\lambda , s}\}$ (where $\Omega _{\mathrm {SL}_2} = \{1, s\}$ ),

• • The trivial representation ${\mathbf {1}}_{\mathrm {SL}_2}$ if $\lambda = 0$ .

By the Künneth rule, cohomological representations of $\mathrm {SL}_2 \times \mathrm {SL}_2(\mathbb R)$ are exactly products of those on $\mathrm {SL}_2(\mathbb R)$ . Those of $H_\infty ^0$ are exactly those of $\mathrm {SL}_2 \times \mathrm {SL}_2(\mathbb R)$ that are trivial on $\pm 1$ – in other words, with $\lambda = a\epsilon _1+b\epsilon _2$ and $a+b$ even.

Consider such $\lambda $ . There are three cases of inductions to consider to compute the cohomological representations of $H_\infty $ . Note that conjugation by $h \in H_\infty -H_\infty ^0$ swaps the two members of a discrete-series L-packet of an embedded $\mathrm {SL}_2$ factor and fixes the trivial representation.

• • $a,b \neq 0$ : We look at the inductions of products of discrete series. This is case (1) so the $4$ products pair up in sums that are $2$ members of an L-packet. These are of course $\pi ^H_{\lambda ,1}$ and $\pi ^H_{\lambda , s}$ , where s is a length- $1$ element of $\Omega _H$ :

  $$ \begin{align*} \pi^H_{\lambda,1}|_{H_\infty^0} &= (\pi_{a \epsilon_1,1} \boxtimes \pi_{b \epsilon_2,1}) \oplus (\pi_{a \epsilon_1,s} \boxtimes \pi_{b \epsilon_2,s}), \\ \pi^H_{\lambda,s}|_{H_\infty^0} &= (\pi_{a \epsilon_1,1} \boxtimes \pi_{b \epsilon_2,s}) \oplus (\pi_{a \epsilon_1,s} \boxtimes \pi_{b \epsilon_2,1}). \end{align*} $$

• • Without loss of generality, $a = 0, b \neq 0$ : We also need to consider inductions of ${\mathbf {1}} \boxtimes \pi _{b \epsilon _2, \star }$ . This is case (1), and both induce to a single irreducible $\sigma ^H_{\lambda }$ :

  $$\begin{align*}\sigma^H_\lambda|_{H_\infty^0} = ({\mathbf{1}} \boxtimes \pi_{b \epsilon_2,1}) \oplus ({\mathbf{1}} \boxtimes \pi_{b \epsilon_2, s}). \end{align*}$$

• • $a = b = 0$ : In addition to both the above, we need to consider the induction of ${\mathbf {1}}_{\mathrm {SL}_2} \boxtimes {\mathbf {1}}_{\mathrm {SL}_2}$ . This is case (2). This trivial representation induces to ${\mathbf {1}}_{H_\infty } \oplus \chi $ on $H_\infty $ . Both factors are cohomological.

Grothendieck group relations stay true restricted to $H_\infty ^0$ , so we can compute traces against $\eta _\lambda $ . Recall that in $\mathrm {SL}_2(\mathbb R)$

$$\begin{align*}{\mathbf{1}} = I - \pi_{0, 1} - \pi_{0,s}, \end{align*}$$

where I is some parabolically induced representation with trivial trace against $\eta ^{\mathrm {SL}_2}_0$ .

First, by our normalization

$$\begin{align*}\operatorname{\mathrm{tr}}_{\pi^H_{\lambda,1}}(\eta^H_\lambda) = \operatorname{\mathrm{tr}}_{\pi^H_{\lambda,s}}(\eta^H_\lambda) = 1/2. \end{align*}$$

Next, working in $H_\infty ^0$ and for $\lambda = b \epsilon _2$ ,

$$\begin{align*}{\mathbf{1}} \boxtimes \pi_{\lambda, \star} = ( I - \pi_{0, 1} - \pi_{0,s}) \boxtimes \pi_{\lambda,\star} = I \boxtimes \pi_{\lambda, \star} - \pi_{0, 1} \boxtimes \pi_{\lambda, \star} - \pi_{0,s} \boxtimes \pi_{\lambda, \star}, \end{align*}$$

so

$$\begin{align*}\sigma^H_\lambda = {\mathbf{1}} \boxtimes \pi_{\lambda, 1} + {\mathbf{1}} \boxtimes \pi_{\lambda, s} = I \boxtimes (\pi_{\lambda, 1} + \pi_{\lambda, s}) - \pi^H_{0 + \lambda,1} - \pi^H_{0 + \lambda, s}, \end{align*}$$

implying

$$\begin{align*}\operatorname{\mathrm{tr}}_{\sigma^H_\lambda}(\eta^H_\lambda) = -1. \end{align*}$$

Finally,

$$ \begin{align*} {\mathbf{1}} \boxtimes {\mathbf{1}} &= ( I - \pi_{0, 1} - \pi_{0,s}) \boxtimes ( I - \pi_{0, 1} - \pi_{0,s}) \\ &= I \boxtimes I - I \boxtimes (\pi_{0,1} + \pi_{0,s}) - (\pi_{0,1} + \pi_{0,s}) \boxtimes I + \pi^H_{0 + 0, 1} + \pi^H_{0+ 0, s}, \end{align*} $$

so

$$\begin{align*}\operatorname{\mathrm{tr}}_{{\mathbf{1}}}(\eta^H_\lambda) = 1. \end{align*}$$

Since $\eta _\lambda $ is supported on $H^0_\infty $ , we similarly have

$$\begin{align*}\operatorname{\mathrm{tr}}_{\chi}(\eta^H_\lambda) = 1. \end{align*}$$

In total, our H-term becomes a count

(8) $$ \begin{align} I^H(\eta^H_\lambda \otimes {\mathbf{1}}_{K_H}) = \sum_{\pi \in \mathcal{AR}_{\mathrm{disc}, \mathrm{ur}}(H)} m_{\mathrm{disc}}^H(\pi) w^H(\pi_\infty), \end{align} $$

where $w^H$ is a weight

$$\begin{align*}w^H(\pi_\infty) = \begin{cases} 0 & \pi_\infty \text{ not cohomological of weight } \lambda \\ 1/2 & \pi_\infty \text{ one of the } \pi^H_{\lambda, *} \\ -1 & \pi_\infty = \sigma^H_\lambda \\ 1 & \pi_\infty \text{ trivial or }\chi\text{ and } \lambda = 0 \end{cases}. \end{align*}$$

Call the cohomological cases type I, II and III in order.

7.2 Reduction to modular form counts

We now recall two results from [Reference Chenevier and RenardCR15]. Consider central isogeny $G \to G'$ of semisimple algebraic groups over $\mathbb Z$ . If $\pi ' = \pi ^{\prime }_\infty \otimes \pi ^{\prime \infty }$ is an unramified, discrete automorphic representation of $G'$ , let $R(\pi ')$ be the set of unitary, admissible representations $\pi = \pi _\infty \otimes \pi ^\infty $ of $G(\mathbb A)$ that satisfy:

• • $\pi ^\infty $ is unramified with set of Satake parameters $c^\infty (\pi ^\infty )$ induced from that of $\pi ^{\prime \infty }$ through $T_{G'}^G : \widehat G' \to \widehat G$ .

• • $\pi _\infty $ is a constituent of the restriction of $\pi ^{\prime }_\infty $ through $G(\mathbb R) \to G'(\mathbb R)$ .

Note that the size of $R(\pi ')$ is the number of constituents of the restriction $\pi ^{\prime }_\infty |_{G(\mathbb R)}$ .

Theorem 7.2.1 [Reference Chenevier and RenardCR15, prop. 4.7]

Let $\pi $ be an automorphic representation of G. Then

$$\begin{align*}m_{\mathrm{disc}}^G(\pi) = \sum_{\pi' : \pi \in R(\pi')} m_{\mathrm{disc}}^{G'}(\pi') [\pi_\infty, \pi^{\prime}_\infty], \end{align*}$$

where $[\pi _\infty , \pi ^{\prime }_\infty ]$ is the multiplicity of $\pi _\infty $ in $\pi ^{\prime }_\infty |_{G(\mathbb R)}$ .

We will apply this with $G = H$ and $G' = \mathrm {PGL}_2 \times \mathrm {PGL}_2$ . Make similar definitions of type I, II and III for representations of $[\mathrm {PGL}_2 \times \mathrm {PGL}_2](\mathbb R)$ . Type I on $\mathrm {PGL}_2 \times \mathrm {PGL}_2$ restricts to the sum over a discrete L-packet on $H_\infty $ . Type II and III on $\mathrm {PGL}_2 \times \mathrm {PGL}_2$ have irreducible restrictions. These restrictions partition the cohomological representations of H except for $\chi $ so

$$\begin{align*}m_{\mathrm{disc}}^H(\pi_\infty \otimes \pi^\infty) = \sum_{c^\infty({\pi'}^\infty) \in (T_{G'}^H)^{-1}(c^\infty(\pi^\infty))} m_{\mathrm{disc}}^{G'}(\pi^{\prime}_\infty \oplus {\pi'}^\infty) \end{align*}$$

when $\pi _\infty \subseteq \pi ^{\prime }_\infty |_{H_\infty }$ and the multiplicity is $0$ when $\pi _\infty = \chi $ . Now, we sum over the constituents of $\pi ^{\prime }_\infty |_{H_\infty }$ and the possible values of $c^\infty (\pi ^\infty )$ , noting that $T_{G'}^H$ is surjective. This gives

Corollary 7.2.2.

$$\begin{align*}\sum_{\pi \in \mathcal{AR}_{\mathrm{disc}, \mathrm{ur}}(H)} m_{\mathrm{disc}}^H(\pi) w^H(\pi_\infty) = \sum_{\pi \in \mathcal{AR}_{\mathrm{disc}, \mathrm{ur}}(G')} m_{\mathrm{disc}}^{G'}(\pi) w^{G'}(\pi_\infty), \end{align*}$$

where $w^{G'}$ is the weight

$$\begin{align*}w^{\mathrm{PGL}_2 \times \mathrm{PGL}_2}(\pi_\infty) = \begin{cases} 1 & \pi_\infty \text{ type I}\\ -1 & \pi_\infty \text{ type II}\\ 1 & \pi_\infty \text{ type III} \end{cases} \end{align*}$$

that only differs from $w^H$ by multiplying the type I case by two.

Let $\mathcal {S}_k(1)$ be the set of normalized, level- $1$ , weight-k cuspidal (new)eigenforms. If $\lambda = a\epsilon _1 + b \epsilon _2$ , then type I representations on $\mathrm {PGL}_2 \times \mathrm {PGL}_2$ correspond to pairs in $S_{a+2}(1) \times S_{b+2}(1)$ . Type II is a single form times the trivial representation, and type III is only the trivial representation.

7.3 Final formula for $S^H$

Therefore, if

$$\begin{align*}S_k = |\mathcal{S}_k(1)|, \end{align*}$$

we get:

(9) $$ \begin{align} I^H(\eta^H_{a \epsilon_1 + b \epsilon_2} \otimes {\mathbf{1}}_{K_H}) = (S_{a+2} - {\mathbf{1}}_{a = 0})(S_{b+2} - {\mathbf{1}}_{b = 0}), \end{align} $$

using canonical measure at finite places. By a classical formula ([Reference Diamond and ShurmanDS05, Thm. 3.5.2], for example),

$$ \begin{align*} S_{a+2} &= \begin{cases} 0 & a+2 = 2 \text{ or } a+2 \text{ odd}\\ \lfloor \frac{a+2}{12} \rfloor - 1 & a+2 \equiv 2\quad \pmod{12} \\ \lfloor \frac{a+2}{12} \rfloor & \text{else} \\ \end{cases}. \end{align*} $$

8.1 First form

Generalizing Equation (4) slightly and substituting in Equations (6) and (7) gives

(10) $$ \begin{align} I^{G_2}(\varphi_{\pi_k} \otimes f^\infty) &= I^{G_2^c}(\eta^{G_2^c}_{(k-2)\beta} \otimes f^\infty) - I^H (\eta^H_{(3k-3)\epsilon_1+ (k-1)\epsilon_2} \otimes (f^\infty)^H) \nonumber\\ &\quad+ I^H (\eta^H_{(3k-2)\epsilon_1 + (k-2)\epsilon_2} \otimes (f^\infty)^H) \end{align} $$

for any unramified function $f^\infty $ (we use here that $(G_2^c)^\infty =(G_2)^\infty $ ). This will let us describe the set $\mathcal {Q}_k(1)$ for $k> 2$ in terms of certain representations of $G_2^c$ and H.

Choose $\pi = \pi _k \otimes \pi ^\infty \in \mathcal {Q}_k(1)$ . Since $\pi ^\infty $ is unramified, it can be described by a sequence of Satake parameters: For each prime p, a semisimple conjugacy class $c_p(\pi ^\infty ) \in [\widehat {G_2}]_{\mathrm {ss}}$ (note that $G_2$ is split so we don’t need to worry about the full Langlands dual and see [Reference Shin and TemplierST16, §3.2] for full background).

The endoscopic datum for H also gives an embedding $\widehat H \hookrightarrow \widehat {G_2}$ (noting again that everything is split) whose image contains a chosen maximal torus and therefore induces a map

$$\begin{align*}T^{G_2}_H : [\widehat H]_{\mathrm{ss}} \twoheadrightarrow [\widehat {G_2}]_{\mathrm{ss}}. \end{align*}$$

The fibers of this map are $\Omega _{G_2}$ -orbits of conjugacy classes in H and have size $3$ at $G_2$ -regular elements.

Proposition 8.1.1. Let $k> 2$ and $\pi ^\infty $ an unramified representation of $(G_2)^\infty $ . Then

$$ \begin{align*} m^{G_2}_{\mathrm{disc}}(\pi_k \otimes \pi^\infty) &= m^{G_2^c}_{\mathrm{disc}}(V_{(k-2)\beta} \otimes \pi^\infty) - \frac12 |S^H(\pi^\infty, (3k-3)\epsilon_1+ (k-1)\epsilon_2)| \\ &\quad + \frac12 |S^H(\pi^\infty, (3k-2)\epsilon_1+ (k-2)\epsilon_2)|. \end{align*} $$

Recall here that $V_\lambda $ is the finite-dimensional representation of $G_2^c$ with highest weight $\lambda $ . Also, $S^H(\pi ^\infty , \lambda )$ is the multiset of $\pi _\infty \otimes \pi _1^\infty \in \mathcal {AR}_{\mathrm {disc}}(H)$ with multiplicity such that

• • $\pi _\infty \in \Pi ^H_{\mathrm {disc}}(\lambda )$ ,

• • For all p, $c_p(\pi _1^\infty ) \in (T^{G_2}_H)^{-1}(c_p(\pi ^\infty ))$ .

Proof. This is a standard Jacquet–Langlands-style argument. Through the Satake isomorphism, each $f_p$ can be thought of as a function $[\widehat {G_2}]_{\mathrm {ss}} \to \mathbb C$ through $f_p(c_p(\pi )) = \operatorname {\mathrm {tr}}_{\pi _p}(f_p)$ . It is in fact a Weyl-invariant regular function on a maximal torus in $\widehat {G_2}$ . The full version of the fundamental lemma (see the introduction to [Reference HalesHal95], for example) shows that

$$\begin{align*}f_p^H(c_p) = f_p(T^{G_2}_H(c_p)) \end{align*}$$

for all $c_p \in \widehat H$ .

There are only finitely many sequences $c_p(\pi _1^\infty )$ and $T^{G_2}_H(c_p(\pi _1^\infty ))$ for $\pi _1^\infty $ the unramified finite component of an automorphic representation either:

• • of $G_2$ with infinite part $\pi _k$ ,

• • of $G_2^c$ with infinite part $V_{(k-2)\beta }$ ,

• • or of H with infinite part in $\Pi _{\mathrm {disc}}((3k-3)\epsilon _1+ (k-1)\epsilon _2)$ or $\Pi _{\mathrm {disc}}((3k-2)\epsilon _1+ (k-2)\epsilon _2)$ .

Therefore, we can choose an $f^\infty $ that is $0$ on all of these sequences $c_p(\pi _1^\infty )$ except $1$ on exactly the sequence $c_p(\pi ^\infty )$ (this reduces to finding Weyl-invariant polynomials on $(\mathbb C^\times )^2$ that take specified values on certain Weyl orbits). The result follows from plugging this $f^\infty $ into Equation (10), noting that the $w^H$ from Equation (8) is always $1/2$ in the relevant cases.

8.2 In terms of modular forms

We can use the argument from Section 7.2 to reduce the H-multiplicity terms to $\mathrm {PGL}_2$ -multiplicity ones.

First, we have a map on conjugacy classes

$$\begin{align*}T^{ H}_{\mathrm{PGL}_2 \times \mathrm{PGL}_2} : [\widehat {\mathrm{PGL}_2 \times \mathrm{PGL}_2}]_{\mathrm{ss}} \twoheadrightarrow [\widehat H]_{\mathrm{ss}}. \end{align*}$$

Since the first group is $\mathrm {SL}_2 \times \mathrm {SL}_2(\mathbb C)$ , the fibers of this map are of the form $\{c, -c\}$ for some $c \in [\mathrm {SL}_2 \times \mathrm {SL}_2(\mathbb C)]_{\mathrm {ss}}$ . Composing then gives map

$$\begin{align*}T^{G_2}_{\mathrm{PGL}_2 \times \mathrm{PGL}_2} : [\widehat {\mathrm{PGL}_2 \times \mathrm{PGL}_2}]_{\mathrm{ss}} \twoheadrightarrow [\widehat{G_2}]_{\mathrm{ss}}. \end{align*}$$

This allows us to define $S^{\mathrm {PGL}_2 \times \mathrm {PGL}_2}(\pi ^\infty , \lambda )$ analogous to $S^H(\pi ^\infty , \lambda )$ for all $\lambda = a \epsilon _1 + b \epsilon _2$ with both a and b even. For indexing purposes, set it to be empty when a and b aren’t even.

Formula (9) also gives us that $S^H(\pi ^\infty , a \epsilon _1 + b \epsilon _2) = \emptyset $ when a and b aren’t both even. Recall from §7.2 that the restriction of discrete series $\pi ^{\mathrm {PGL}_2 \times \mathrm {PGL}_2}_\lambda $ to $H(\mathbb R)$ has as components the two members of the L-packet $\Pi _{\mathrm {disc}}^H(\lambda )$ . Therefore, a similar analysis using Theorem 7.2.1 shows that

$$\begin{align*}|S^H(\pi^\infty, \lambda)| = 2|S^{\mathrm{PGL}_2 \times \mathrm{PGL}_2}(\pi^\infty, \lambda)|. \end{align*}$$

Finally, $\mathrm {PGL}_2$ is a quotient of $\mathrm {GL}_2$ by a central torus with trivial Galois cohomology, so automorphic representations on $\mathrm {PGL}_2$ are just those on $\mathrm {GL}_2$ with all components having trivial central character. Recalling injection

$$\begin{align*}\iota : [\mathrm{SL}_2 \times \mathrm{SL}_2(\mathbb C)]_{\mathrm{ss}} \hookrightarrow [\mathrm{GL}_2 \times \mathrm{GL}_2(\mathbb C)]_{\mathrm{ss}}, \end{align*}$$

this gives:

Corollary 8.2.1. Let $k> 2$ and $\pi ^\infty $ an unramified representation of $(G_2)^\infty $ . Then

$$ \begin{align*} m^{G_2}_{\mathrm{disc}}(\pi_k \otimes \pi^\infty) = m^{G_2^c}_{\mathrm{disc}}(V_{(k-2)\beta} \otimes \pi^\infty) - |S^{\mathrm{GL}_2 \times \mathrm{GL}_2}(\pi^\infty, (3k-3)\epsilon_1+ (k-1)\epsilon_2)| \\ + |S^{\mathrm{GL}_2 \times \mathrm{GL}_2}(\pi^\infty, (3k-2)\epsilon_1+ (k-2)\epsilon_2)|. \end{align*} $$

Recall here that $V_\lambda $ is the finite-dimensional representation of $G_2^c$ with highest weight $\lambda $ . Also, $S^{\mathrm {GL}_2 \times \mathrm {GL}_2}(\pi ^\infty , \lambda )$ is the set of $\pi _\infty \otimes \pi _1^\infty \in \mathcal {AR}_{\mathrm {disc}}(\mathrm {GL}_2 \times \mathrm {GL}_2)$ such that

• • $\pi _\infty $ is the discrete series $\pi ^{\mathrm {GL}_2 \times \mathrm {GL}_2}_\lambda $ ,

• • For all p, $c_p(\pi _1^\infty ) = \iota (c^{\prime }_p)$ for some $c^{\prime }_p \in (T^{G_2}_{\mathrm {PGL}_2 \times \mathrm {PGL}_2})^{-1}(c_p(\pi ^\infty ))$ . Here $\iota $ is the map $ [\mathrm {SL}_2 \times \mathrm {SL}_2(\mathbb C)]_{\mathrm {ss}} \hookrightarrow [\mathrm {GL}_2 \times \mathrm {GL}_2(\mathbb C)]_{\mathrm {ss}}$ .

Of course, since all infinite factors in sight are discrete series, we may again replace the $m_{\mathrm {disc}}$ by $m_{\mathrm {cusp}}$ using [Reference WallachWal84].

Note of course that $S^{\mathrm {GL}_2 \times \mathrm {GL}_2}(\pi ^\infty , a\epsilon _1 + b \epsilon _2) = \emptyset $ unless both a and b are even. Therefore, we can interpret this as, for $k> 2$ :

• • If k is even: $\mathcal {Q}_k(1)$ is the corresponding set of representations transferred from $G_2^c$ in addition to representations transferred from pairs of cuspidal eigenforms in $\mathcal {S}_{3k}(1) \times \mathcal {S}_{k}(1)$ .

• • If k is odd: $\mathcal {Q}_k(1)$ is the corresponding set of representations transferred from $G_2^c$ except for representations that are also transferred from pairs of cuspidal eigenforms in $\mathcal {S}_{3k-1}(1) \times \mathcal {S}_{k+1}(1)$ .

Results for level $>1$ would be a lot more complicated since formula (4) would have many further hyperendoscopic terms and the comparison to $\mathrm {GL}_2 \times \mathrm {GL}_2$ would not work as nicely.

9.1 Formula in terms of $I^{G_2^c}$

To get counts instead of a list, combining formulas (2),(10) and (9) gives that

(11) $$ \begin{align} |\mathcal{Q}_k(1)| &= I^{G_2^c}(\eta^{G_2^c}_\lambda \otimes {\mathbf{1}}_{K^\infty_{G_2^c}}) - (S_{3k-1} -{\mathbf{1}}_{3k-3 = 0})(S_{k+1} -{\mathbf{1}}_{k-1 = 0}) \nonumber\\&\quad+ (S_{3k} -{\mathbf{1}}_{3k-2 = 0})(S_{k} -{\mathbf{1}}_{k-2 = 0}), \end{align} $$

where $S_k$ as before represents the count of classical modular forms of weight k.

Substituting in the formulas for $S_k$ , for $k> 2$ :

$$ \begin{align*} |\mathcal{Q}_k(1)| &= I^{G_2^c}(\eta_\lambda \otimes {\mathbf{1}}_{K^\infty_{G_2^c}}) \\ &\qquad + \begin{cases} \lfloor \frac k4 \rfloor \left( \lfloor \frac k{12} \rfloor - 1 \right) & k \equiv 2\quad \pmod{12} \\ \lfloor \frac k4 \rfloor \lfloor \frac k{12} \rfloor & k \equiv 0,4,6,8,10\quad \pmod{12} \\ -\left(\lfloor \frac {3k-1}{12} \rfloor - 1\right) \left( \lfloor \frac {k+1}{12} \rfloor - 1 \right) & k \equiv 1\quad \pmod{12} \\ -\left(\lfloor \frac {3k-1}{12} \rfloor - 1\right) \lfloor \frac {k+1}{12} \rfloor & k \equiv 5,9\quad \pmod{12} \\ -\lfloor \frac {3k-1}{12} \rfloor \lfloor \frac {k+1}{12} \rfloor & k \equiv 3,7,11\quad \pmod{12} \\ \end{cases}. \end{align*} $$

9.2 Computing $I^{G_2^c}$

The group $G_2^c(\mathbb R)$ is compact, so the $I^{G_2^c}$ term takes a very simple form: $L^2(G_2^c(\mathbb Q) \backslash G_2^c(\mathbb A))$ decomposes as a direct sum of automorphic representations and the EP-functions $\eta _\lambda $ are just scaled matrix coefficients of the finite-dimensional representations $V_\lambda $ with highest weight $\lambda $ on $G_2^c(\mathbb R)$ . Therefore,

$$\begin{align*}I^{G_2^c}(\eta_\lambda \otimes {\mathbf{1}}_{K^\infty_{G_2^c}}) = \sum_{\pi \in \mathcal{AR}(G_2^c)} {\mathbf{1}}_{\pi_\infty = V_\lambda} \operatorname{\mathrm{tr}}_{\pi^\infty}({\mathbf{1}}_{K^\infty_{G_2^c}}), \end{align*}$$

which is just counting the number of unramifed automorphic reps of $G_2^c$ that have infinite component $V_\lambda $ .

For reader convenience, we now explain in detail an argument well known to experts. Since unramified representations have one-dimensional spaces of $K^\infty $ -fixed vectors, taking $K^\infty _{G_2^c}$ invariants sends each such $\pi $ to a linearly independent copy of $V_\lambda $ that together span the $V_\lambda $ -isotypic component of

$$\begin{align*}L^2(G_2^c(\mathbb Q) \backslash G_2^c(\mathbb A) / K^\infty_{G_2^c}) = L^2(G_2^c(\mathbb Z) \backslash G_2^c(\mathbb R)) \subseteq L^2(G_2^c(\mathbb R)). \end{align*}$$

By Peter–Weyl, $L^2(G_2^c(\mathbb R))$ has $V_\lambda $ -isotypic component $V_\lambda ^{\oplus \dim V_\lambda }$ . In fact, this component for both the left and right actions is the same subspace. Therefore, the number of copies of $V_\lambda \subseteq L^2(G_2^c(\mathbb Z) \backslash G_2^c(\mathbb R))$ is $\dim \left (V_\lambda ^{G_2^c(\mathbb Z)}\right )$ by a dimension count.

Summarizing:

(12) $$ \begin{align} I^{G_2^c}(\eta_\lambda \otimes {\mathbf{1}}_{K^\infty_{G_2^c}}) = \dim \left(V_\lambda^{G_2^c(\mathbb Z)}\right). \end{align} $$

A PARI/GP 2.5.0 program in the online appendix to [Reference Chenevier and RenardCR15] computes this for all $\lambda $ by pairing the trace character of $V_\lambda |_{G_2(\mathbb Z)}$ with the trivial character.

An explicit paper formula for this computation is more-or-less written out in an honors thesis of Steven Sullivan [Reference SullivanSul13]. Sullivan writes out the traces of all $16$ conjugacy classes in $G_2^c(\mathbb Z)$ against $V_{(k-2) \beta }$ as polynomials of k with coefficients that are sums of kth powers of $7$ th, $8$ th and $12$ th roots of unity. This gets a polynomial expression for the trace character pairing and therefore $I^{G_2^c}(\eta _\lambda \otimes {\mathbf {1}}_{K^\infty _{G_2^c}})$ in cases $\pmod {168}$ . Simplifications in Mathematica give a reasonable closed-form version in Section 9.4.

It is important to note here that getting the explicit descriptions and sizes of the conjugacy classes in $G_2^c(\mathbb Z)$ was nontrivial and required some trickery in both Sullivan’s and Chenevier-Taïbi’s computations. This step would be an obstacle to any generalizations.

9.3 Table of counts

Table 1 gives values of $|\mathcal {Q}_k(1)|$ for $k=3$ to $52$ produced by formula (11) and [Reference Chenevier and RenardCR15]’s table for formula (12). The lowest-weight example is bolded, although this work does not rule out the existence of an example with weight $2$ or weight $1$ (as defined by [Reference PollackPol20, §1.1]).

Table 1 Counts of discrete, quaternionic automorphic representations of level $1$ on $G_2$ .

9.4 Explicit formula

For the reader’s amusement, we build off the work of [Reference SullivanSul13] to present a closed-form formula for $|\mathcal {Q}_k(1)|$ that fits in a few lines:

$$ \begin{align*} &|\mathcal{Q}_{n+2}(1)| = \\ & \frac1{12096} \frac1{120}(n+1)(3n+4)(n+2)(3n+5)(2n+3) + \frac1{216}\frac16 (n+1) (n+ 2 ) (2 n+3) \\ &+ \frac5{192}\frac18 \begin{cases} (n+2) ( 3 n+4) & n = 0\quad \pmod 2 \\ -(n+1) (3 n+5)& n = 1 \quad\pmod 2 \\ \end{cases} + \frac1{18}\begin{cases} \frac{2n}3 + 1& n = 0 \quad\pmod 3 \\ - \lfloor \frac n3 \rfloor - 1 & n = 1,2 \quad\pmod 3 \end{cases} \\ &+ \frac1{32}\begin{cases} \frac{3n}2 + 10& n = 0 \quad\pmod 4 \\ 6\lfloor \frac{n}4 \rfloor - 4 & n = 1 \quad\pmod 4 \\ - 2\lfloor \frac{n}4 \rfloor - 2 & n = 2,3 \quad\pmod 4 \end{cases} + \frac1{24} \begin{cases} 3\lfloor \frac{n}6 \rfloor+ 5& n = 0,1 \quad\pmod 6 \\ 3 \lfloor \frac{n}6 \rfloor -2 & n = 2,3 \quad\pmod 6 \\ 3 \lfloor \frac{n}6 \rfloor +3 & n = 4,5 \quad\pmod 6 \end{cases} \\ &+ \frac17 \begin{cases} 1 & n = 0 \quad\pmod 7 \\ -1 & n = 4 \quad\pmod 7 \\ 0 & n = 1,2,3,5,6 \quad\pmod 7 \end{cases} + \frac14\begin{cases} 1 & n = 0 \quad\pmod 8 \\ -1 & n = 5 \quad\pmod 8 \\ 0 & n = 1,2,3,4,6,7 \quad\pmod 8 \end{cases} \\ &+ \begin{cases} \lfloor \frac {n+2}4 \rfloor \left( \lfloor \frac {n+2}{12} \rfloor - 1 \right) & n = 0 \quad\pmod{12} \\ \lfloor \frac {n+2}4 \rfloor \lfloor \frac {n+2}{12} \rfloor & n = 2,4,6,8,10 \quad\pmod{12} \\ -\left(\lfloor \frac {3n+5}{12} \rfloor - 1\right) \left( \lfloor \frac {n+3}{12} \rfloor - 1 \right) & n = 11 \quad\pmod{12} \\ -\left(\lfloor \frac {3n+5}{12} \rfloor - 1\right) \lfloor \frac {n+3}{12} \rfloor & n = 3,7 \quad\pmod{12} \\ -\lfloor \frac {3n + 5}{12} \rfloor \lfloor \frac {n+3}{12} \rfloor & n = 1,5,9 \quad\pmod{12} \\ \end{cases}. \end{align*} $$