English

Noncommutative Abel-like identities

Combinatorics 2026-04-15 v1

Abstract

We generalize the Abel--Hurwitz identities to an almost entirely noncommutative setting. Namely, let VV be a finite set of size nn, and let L\mathbb{L} be any noncommutative ring. For each sVs\in V, let xsLx_{s}\in\mathbb{L}. Set x(S):=sSxsx\left( S\right) :=\sum_{s\in S}x_{s} for any SVS\subseteq V. Let XX and YY be two elements of L\mathbb{L} such that X+YX+Y lies in the center of L\mathbb{L}. Then, we show that% \begin{align*} & \sum_{S\subseteq V}\left( X+x\left( S\right) \right) ^{\left\vert S\right\vert }\left( Y-x\left( S\right) \right) ^{n-\left\vert S\right\vert }=\sum_{\substack{i_{1},i_{2},\ldots,i_{k}\in V\text{ distinct}% }}\left( X+Y\right) ^{n-k}x_{i_{1}}x_{i_{2}}\cdots x_{i_{k}};\\ & \sum_{S\subseteq V}X\left( X+x\left( S\right) \right) ^{\left\vert S\right\vert -1}\left( Y-x\left( S\right) \right) ^{n-\left\vert S\right\vert }=\left( X+Y\right) ^{n};\\ & \sum_{S\subseteq V}X\left( X+x\left( S\right) \right) ^{\left\vert S\right\vert -1}\left( Y-x\left( S\right) \right) ^{n-\left\vert S\right\vert -1}\left( Y-x\left( V\right) \right) =\left( X+Y-x\left( V\right) \right) \left( X+Y\right) ^{n-1}. \end{align*} (Negative powers are understood to be cancelled by other factors.)

Keywords

Cite

@article{arxiv.2604.12619,
  title  = {Noncommutative Abel-like identities},
  author = {Darij Grinberg},
  journal= {arXiv preprint arXiv:2604.12619},
  year   = {2026}
}

Comments

21 pages. Preprint from 2017, with minor corrections. Applications and other comments welcome!

R2 v1 2026-07-01T12:08:36.917Z