Rational invariants of even ternary forms under the orthogonal group

Abstract : In this article we determine a generating set of rational invariants of minimal cardinality for the action of the orthogonal group O3 on the space $R[x, y, z]_2d$ of ternary forms of even degree 2d. The construction relies on two key ingredients: On one hand, the Slice Lemma allows us to reduce the problem to determining the invariants for the action on a subspace of the finite subgroup B3 of signed permutations. On the other hand, our construction relies in a fundamental way on specific bases of harmonic polynomials. These bases provide maps with prescribed B3-equivariance properties. Our explicit construction of these bases should be relevant well beyond the scope of this paper. The expression of the B3-invariants can then be given in a compact form as the composition of two equivariant maps. Instead of providing (cumbersome) explicit expressions for the O3-invariants, we provide efficient algorithms for their evaluation and rewriting. We also use the constructed B3-invariants to determine the O3-orbit locus and provide an algorithm for the inverse problem of finding an element in $R[x, y, z]_2d$ with prescribed values for its invariants. These are the computational issues relevant in brain imaging.
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Contributor : Evelyne Hubert <>
Submitted on : Sunday, July 8, 2018 - 12:47:19 PM
Last modification on : Monday, August 20, 2018 - 9:46:01 AM


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  • HAL Id : hal-01570853, version 3


Paul Görlach, Evelyne Hubert, Théo Papadopoulo. Rational invariants of even ternary forms under the orthogonal group. 2018. ⟨hal-01570853v3⟩



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