Mathematics – Algebraic Geometry
Scientific paper
2002-04-02
Contemporary Mathematics 324 (2003), Topics in Algebraic Geometry and Noncommutative Geometry, C. G. Melles, J.-P. Brasselet,
Mathematics
Algebraic Geometry
Submitted to the Proceedings of the Colloquium in the memory of Ruth Michler (Luminy 2001)
Scientific paper
We consider the weak version of the Deligne-Simpson problem: give necessary and sufficient conditions upon the conjugacy classes $c_j\subset gl(n,{\bf C})$ (resp. $C_j\subset GL(n,{\bf C})$) so that there exist $(p+1)$-tuples of matrices $A_j\in c_j$, $A_1+... +A_{p+1}=0$ (resp. $M_1... M_{p+1}=I$) with trivial centralizers (i.e. reduced to scalars). The true Deligne-Simpson problem requires irreducibility instead of triviality of the centralizer. When the eigenvalues are generic, a Criterium on the Jordan normal forms defined by the conjugacy classes gives the necessary and sufficient conditions for solvability of the true problem. For index of rigidity 2 (i.e. when the sum of the dimensions of the conjugacy classes equals $2n^2-2$) we show that for a sufficiently large class of $(p+1)$-tuples of conjugacy classes the answer to the weak problem is negative. These conjugacy classes define Jordan normal forms that satisfy the Criterium.
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