English

The generic canonical form for $^\star$congruence of matrices

Spectral Theory 2025-12-16 v1

Abstract

First, we prove that the set of n×nn\times n complex matrices is the closure of a certain open subset whose elements have a very specific canonical form under congruence, which is uniquely determined up to the values of some parameters, but which has a slightly different expression depending on whether nn is even or odd. As a consequence, the canonical form under congruence of the elements of this subset can be considered the generic canonical form under congruence of complex n×nn\times n matrices. Second, we prove that the set of n×nn\times n complex matrices is the union of the closures of certain n/2+1\lfloor n/2\rfloor+1 open subsets and that, for each of these subsets, its elements have a very specific canonical form under ^*congruence, which is uniquely determined up to the values of some parameters. As a consequence, the n/2+1\lfloor n/2\rfloor+1 canonical forms under ^*congruence of the elements of each of these subsets can be considered the generic canonical forms under ^*congruence of complex n×nn\times n matrices. So, there is only one generic canonical form under congruence whereas the number of generic canonical forms under ^*congruence is n/2+1\lfloor n/2\rfloor+1 instead, which reveals a strong dichotomy between the relations of congruence and ^*congruence with respect to generic structures. In other words, we determine in this paper the generic matrix representations of n×nn\times n bilinear and sesquilinear forms in Cn×Cn\mathbb{C}^n \times \mathbb{C}^n.

Keywords

Cite

@article{arxiv.2512.12407,
  title  = {The generic canonical form for $^\star$congruence of matrices},
  author = {Fernando De Terán and Froilán M. Dopico},
  journal= {arXiv preprint arXiv:2512.12407},
  year   = {2025}
}