PreprintCC BY 4.0arXiv
On Some Lie Group Invariant noncommutative functions
Abstract
We study noncommutative polynomials and functions invariant under linear actions of infinite (compact Lie) groups, with a focus on SU(2) acting on two noncommuting variables. For SU(2), we give a complete description of the invariant noncommutative polynomials: there are no invariants of odd degree, and in degree 2n the invariants are exactly the span of the S2n-orbit of (XY−YX)n. We also construct an explicit basis recursively from the operation (A,B)↦ XAYB−YAXB, and show that the space of degree-2n invariants has dimension equal to the n-th Catalan number. All proofs proceed by direct computation with the Haar integral.
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BibTeX
@misc{lu2026some,
title = {{On Some Lie Group Invariant noncommutative functions}},
author = {Jiachen Lu},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37185},
eprint = {2610.37185},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37185},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - On Some Lie Group Invariant noncommutative functions
AU - Jiachen Lu
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37185
UR - https://arxiv.org/abs/2610.37185
AB - We study noncommutative polynomials and functions invariant under linear actions of infinite (compact Lie) groups, with a focus on SU(2) acting on two noncommuting variables. For SU(2), we give a complete description of the invariant noncommutative polynomials: there are no invariants of odd degree, and in degree 2n the invariants are exactly the span of the S_2n-orbit of (XY−YX)^n. We also construct an explicit basis recursively from the operation (A,B)↦ XAYB−YAXB, and show that the space of degree-2n invariants has dimension equal to the n-th Catalan number. All proofs proceed by direct computation with the Haar integral.
ER -