Mathematics – Representation Theory
Scientific paper
2006-11-07
Linear Algebra and its Applications 428 (2008), 742-753
Mathematics
Representation Theory
12 pages, (v2) revision, to appear in Linear Algebra and its Applications
Scientific paper
10.1016/j.laa.2007.08.002
We show that for piecewise hereditary algebras, the periodicity of the Coxeter transformation implies the non-negativity of the Euler form. Contrary to previous assumptions, the condition of piecewise heredity cannot be omitted, even for triangular algebras, as demonstrated by incidence algebras of posets. We also give a simple, direct proof, that certain products of reflections, defined for any square matrix A with 2 on its main diagonal, and in particular the Coxeter transformation corresponding to a generalized Cartan matrix, can be expressed as $-A_{+}^{-1} A_{-}^t$, where A_{+}, A_{-} are closely associated with the upper and lower triangular parts of A.
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