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On Trace for Modules

Published online by Cambridge University Press:  22 January 2016

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Classically, trace was defined as the sum of the diagonal entries of a square matrix with entries in a field. This notion played an important role in classical mathematics, e.g. in the theory of algebras over a field of characteristic zero, and in the theory of group characters (as in [1]). A generalization to endomorphisms of a finitely generated projective module over any ring R with unit is well-known. For such a module P the canonical homomorphism Ψ: P* ⊗ P→ EndR(P) is an isomorphism. Then the composite ε˚Ψ-1: EndR(P) → P* ↗ P→ R, where e denotes “evaluation”, is a homomorphism which coincides with the classical trace whenever P is free. This version of trace has been used by Hattori [3] and others to study projective modules. However, this approach to trace is limited to the finitely generated projective modules, since it can be shown that Ψ is an isomorphism if and only if P is finitely generated and projective.

Type
Research Article
Copyright
Copyright © Editorial Board of Nagoya Mathematical Journal 1970

References

[1] Curtis, C.W., Reiner, I., Representation Theory of Finite Groups and Associative Algebras. Interscience Publ., New York (1962).Google Scholar
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[3] Hattori, A., Rank Element of a Projective Module, Nagoya Math J. 25(1965), 113120.Google Scholar
[4] Nagata, M., Local Rings, Interscience Publ., New York (1962).Google Scholar
[5] Röhrl, H., A Categorical Setting for Traces and Determinants, to appear Nagoya Math J., 34 (1968).Google Scholar
[6] Zariski, O., Samuel, P., Commutative Algebra vol. II, D. Van Nostrand Co., Inc., Princeton, 1960.Google Scholar