A well-known result, due to Ostrowski, states that if ${{\left\| P-Q \right\|}_{2}}\,<\,\varepsilon $, then the roots $({{x}_{j}})$ of $P$ and $({{y}_{j}})$ of $Q$ satisfy $\left| {{x}_{j}}\,-\,{{y}_{j}} \right|\,\le \,Cn{{\varepsilon }^{1/n}}$, where $n$ is the degree of $P$ and $Q$. Though there are cases where this estimate is sharp, it can still be made more precise in general, in two ways: first by using Bombieri’s norm instead of the classical ${{l}_{1}}$ or ${{l}_{2}}$ norms, and second by taking into account the multiplicity of each root. For instance, if $x$ is a simple root of $P$, we show that $\left| x\,-\,y \right|\,<\,C\varepsilon $ instead of ${{\varepsilon }^{1/n}}$. The proof uses the properties of Bombieri’s scalar product andWalsh Contraction Principle.

A $Q$-set is a set of reals every subset of which is a relative ${{G}_{\delta }}$. We investigate the combinatorics of $Q$-sets and discuss a question of Miller and Zhou on the size $q$ of the smallest set of reals which is not a $Q$-set. We show in particular that various natural lower bounds for $q$ are consistently strictly smaller than $q$.

Analogues of van der Waerden’s theorem on arithmetic progressions are considered where the family of all arithmetic progressions, $\text{AP}$, is replaced by some subfamily of $\text{AP}$. Specifically, we want to know for which sets $A$, of positive integers, the following statement holds: for all positive integers $r$ and $k$, there exists a positive integer $n={w}'\text{(}k,r)$ such that for every $r$-coloring of $[1,\,n]$ there exists a monochromatic $k$-term arithmetic progression whose common difference belongs to $A$. We will call any subset of the positive integers that has the above property large. A set having this property for a specific fixed $r$ will be called $r$-large. We give some necessary conditions for a set to be large, including the fact that every large set must contain an infinite number of multiples of each positive integer. Also, no large set $\{{{a}_{n}}\,:\,n\,=\,1,\,2,\ldots \}$ can have $\underset{n\to \infty }{\mathop{\lim \,\inf }}\,\,\frac{{{a}_{n+1}}}{{{a}_{n}}}\,>\,1$. Sufficient conditions for a set to be large are also given. We show that any set containing $n$-cubes for arbitrarily large $n$, is a large set. Results involving the connection between the notions of “large” and “2-large” are given. Several open questions and a conjecture are presented.

Recent work of Ding and Huang shows that if we perturb a bounded operator (between Hilbert spaces) which has closed range, then the perturbed operator again has closed range. We extend this result by introducing a weaker perturbation condition, and our result is then used to prove a theorem about the stability of frames for a subspace.

It is shown that under $\text{ZFC}$ almost all planar compacta that meet every line in at most two points are subsets of sets that meet every line in exactly two points. This result was previously obtained by the author jointly with K. Kunen and J. vanMill under the assumption that Martin’s Axiom is valid.

If $V\,\to \,X$ is a vector bundle of fiber dimension $k$ and $Y\,\to \,X$ is a finite sheeted covering map of degree $d$, the implications for the Euler class $e(V)$ in ${{H}^{k}}(X)$ of $V$ implied by the existence of an embedding $Y\,\to \,V$ lifting the covering map are explored. In particular it is proved that $d{{d}^{\prime }}\text{e(V)}\text{=}\text{0}$ where ${{d}^{\prime }}$ is a certain divisor of $d\,-\,1$, and often ${{d}^{\prime }}=1$.

Let ${{j}_{v,1}}$ be the smallest (first) positive zero of the Bessel function ${{J}_{v}}(z),\,v\,>\,-\,1$, which becomes zero when $v$ approaches −1. Then $j_{v,1}^{2}$ can be continued analytically to $-2\,<\,v\,<\,-1$, where it takes on negative values. We show that $j_{v,1}^{2}$ is a convex function of $v$ in the interval $-2\,<\,v\,\le \,0$, as an addition to an old result [Á. Elbert and A. Laforgia, SIAM J. Math. Anal. 15(1984), 206–212], stating this convexity for $v\,>\,0$. Also the monotonicity properties of the functions $\frac{j_{v,1}^{2}}{4(v+1)},\,\frac{j_{v,1}^{2}}{4(v+1)\sqrt{v+2}}$ are determined. Our approach is based on the series expansion of Bessel function ${{J}_{v}}(z)$ and it turned out to be effective, especially when $-2\,<\,v\,<\,-1$.

We consider the asymptotic behavior of the moments of the sum-of-digits function of canonical number systems in number fields. Using Delange’s method we obtain the main term and smaller order terms which contain periodic fluctuations.

We study the splitting of Fermat Jacobians of prime degree $\ell $ over an algebraic closure of a finite field of characteristic $p$ not equal to $\ell $. We prove that their decomposition is determined by the residue degree of $p$ in the cyclotomic field of the $\ell $-th roots of unity. We provide a numerical criterion that allows to compute the absolutely simple subvarieties and their multiplicity in the Fermat Jacobian.

Let ${{A}_{i}},\,{{B}_{i}}$ and ${{X}_{i}}\,(i\,=\,1,\,2,\ldots ,\,n)$ be operators on a separable Hilbert space. It is shown that if $f$ and $g$ are nonnegative continuous functions on $\left[ 0,\infty \right)$ which satisfy the relation $f\,(t)g(t)\,=\,t$ for all $t$ in $\left[ 0,\infty \right)$, then

$${{\left\| \left| \,{{\left| \sum\limits_{i=1}^{n}{A_{i}^{*}{{X}_{i}}{{B}_{i}}} \right|}^{r}} \right| \right\|}^{2}}\,\le \,\left\| \left| {{\left( \sum\limits_{i=1}^{n}{A_{i}^{*}f{{(\left| X_{i}^{*} \right|)}^{2}}{{A}_{i}}} \right)}^{r}} \right| \right\|\,\,\left\| \left| {{\left( \sum\limits_{i=1}^{n}{B_{i}^{*}g{{(\left| {{X}_{i}} \right|)}^{2}}\,{{B}_{i}}} \right)}^{r}} \right| \right\|$$

for every $r\,>\,0$ and for every unitarily invariant norm. This result improves some known Cauchy-Schwarz type inequalities. Norm inequalities related to the arithmetic-geometric mean inequality and the classical Heinz inequalities are also obtained.

Let $B\,=\,{{B}_{n}}$ be the open unit ball of ${{C}^{n}}$ with volume measure $v$, $U\,=\,{{B}_{1}}$ and $B$ be the Bloch space on $U.\,{{\mathcal{A}}^{2,\alpha }}(B)$, $1\,\le \,\alpha \,<\infty $, is defined as the set of holomorphic $f\,:\,B\,\to \,C$ for which

$$\int_{B\,}{{{\left| f(z) \right|}^{2}}}{{\left( \frac{1}{\left| z \right|}\,\log \frac{1}{1\,-\left| z \right|} \right)}^{-\alpha }}\,\frac{dv(z)}{1\,-\,\left| z \right|}\,<\,\infty $$

if $0\,<\,\alpha \,<\infty $ and ${{\mathcal{A}}^{2,1}}(B)\,=\,{{H}^{2}}(B)$, the Hardy space. Our objective of this note is to characterize, in terms of the Bergman distance, those holomorphic $f\,:\,B\,\to \,U$ for which the composition operator ${{C}_{f}}\,:\,B\,\to \,{{\mathcal{A}}^{2,\alpha }}(B)$ defined by ${{C}_{f}}(g)=g\,\text{o}\,f\text{,}\,g\in B$, is bounded. Our result has a corollary that characterize the set of analytic functions of bounded mean oscillation with respect to the Bergman metric.

We consider some geometric properties of eigenvectors of linear operators on infinite dimensional Hilbert space. It is proved that the property of a family of vectors $({{x}_{\text{n}}})$ to be eigenvectors $T{{x}_{n}}={{\lambda }_{n}}{{x}_{n}}({{\lambda }_{n}}\ne {{\lambda }_{k}}\text{for}\,n\ne k)$ of a bounded operator $T$ (admissibility property) is very instable with respect to additive and linear perturbations. For instance, (1) for the sequence ${{\left( {{x}_{n}}+{{\epsilon }_{n}}{{v}_{n}} \right)}_{n\ge k(\epsilon )}}$ to be admissible for every admissible $({{x}_{\text{n}}})$ and for a suitable choice of small numbers ${{\epsilon }_{n}}\,\ne \,0$ it is necessary and sufficient that the perturbation sequence be eventually scalar: there exist ${{\text{ }\!\!\gamma\!\!\text{ }}_{n}}\,\in \,C$ such that ${{v}_{n}}\,=\,{{\gamma }_{n}}{{v}_{k}}\,\text{for}\,n\,\ge \,\text{k}$ (Theorem 2); (2) for a bounded operator $A$ to transform admissible families $({{x}_{\text{n}}})$ into admissible families $(A{{x}_{n}})$ it is necessary and sufficient that $A$ be left invertible (Theorem 4).

For a compact Hausdorff space with a dense set of isolated points, we give a complete description of points of weak$^{*}$-norm continuity in the dual unit ball of the space of Banach space valued functions that are continuous when the range has the weak topology. As an application we give a complete description of points of weak-norm continuity of the unit ball of the space of vector measures when the underlying Banach space has the Radon-Nikodym property.

Vector invariants of finite groups (see the introduction for an explanation of the terminology) have often been used to illustrate the difficulties of invariant theory in the modular case: see, e.g., [1], [2], [4], [7], [11] and [12]. It is therefore all the more surprising that the unpleasant properties of these invariants may be derived from two unexpected, and remarkable, nice properties: namely for vector permutation invariants of the cyclic group $Z/p$ of prime order in characteristic $p$ the image of the transfer homomorphism $\text{T}{{\text{r}}^{Z/p}}\,:\,F[V]\,\to \,F{{[V]}^{Z/p}}$ is a prime ideal, and the quotient algebra $F{{[V]}^{Z/p}}/\,\text{IM(T}{{\text{r}}^{Z/p}})$ is a polynomial algebra on the top Chern classes of the action.