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Ideal decompositions and computation of tensor normal forms

B. Fiedler
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Abstract

Symmetry properties of r-times covariant tensors T can be described by certain linear subspaces W of the group ring K[S_r] of a symmetric group S_r. If for a class of tensors T such a W is known, the elements of the orthogonal subspace W^{\bot} of W within the dual space of K[S_r] yield linear identities needed for a treatment of the term combination problem for the coordinates of the T. We give the structure of these W for every situation which appears in symbolic tensor calculations by computer. Characterizing idempotents of such W can be determined by means of an ideal decomposition algorithm which works in every semisimple ring up to an isomorphism. Furthermore, we use tools such as the Littlewood-Richardson rule, plethysms and discrete Fourier transforms for S_r to increase the efficience of calculations. All described methods were implemented in a Mathematica package called PERMS.

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B. Fiedler. 2002-11-09. Ideal decompositions and computation of tensor normal forms. https://arxiv.org/abs/math/0211156

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