Dimensions of permanental varieties in arbitrary size
Abstract
Let Ih(a,b) be the ideal generated by the h× h permanents of a generic a× b matrix over a field of characteristic different from two. We prove that dim K[X]/Ih(a,b)=max(a,b)(h−1) whenever 1≤ h≤min(a,b) and h<max(a,b). Consequently, the critical locus of the permanent of order n has codimension 2n for every n≥ 2. We also prove that the maximal permanents of a generic k×(k+1) matrix generate a geometrically reduced complete intersection. The dimension argument uses a pivot expansion and cubic relations in the border variables. Two successive comparisons with highest-degree equations reduce the relevant fibers to a coordinate-support calculation. Reducedness follows from a separate regular-sequence argument and a dimension estimate excluding components over the exceptional locus.
Versions
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BibTeX
@misc{xie2026dimensions,
title = {{Dimensions of permanental varieties in arbitrary size}},
author = {Ying Xie},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.27320},
eprint = {2609.27320},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.27320},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Dimensions of permanental varieties in arbitrary size
AU - Ying Xie
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.27320
UR - https://arxiv.org/abs/2609.27320
AB - Let I_h(a,b) be the ideal generated by the h× h permanents of a generic a× b matrix over a field of characteristic different from two. We prove that dim K[X]/I_h(a,b)=max(a,b)(h−1) whenever 1≤ h≤min(a,b) and h<max(a,b). Consequently, the critical locus of the permanent of order n has codimension 2n for every n≥ 2. We also prove that the maximal permanents of a generic k×(k+1) matrix generate a geometrically reduced complete intersection. The dimension argument uses a pivot expansion and cubic relations in the border variables. Two successive comparisons with highest-degree equations reduce the relevant fibers to a coordinate-support calculation. Reducedness follows from a separate regular-sequence argument and a dimension estimate excluding components over the exceptional locus.
ER -