The cohomology of the nilCoxeter algebra
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arXiv
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| Format: | Preprint |
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2024
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| _version_ | 1866908827782217728 |
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| author | Benson, David J. |
| author_facet | Benson, David J. |
| contents | The nilCoxeter algebra $\mathcal{N}S_n$ of the symmetric group $S_n$ is the algebra over $\mathbb{Z}$ with generators $Y_i$ ($1\leqslant i\leqslant n-1$), satisfying the braid relations $Y_iY_{i+1}Y_i=Y_{i+1}Y_iY_{i+1}$, $Y_iY_j=Y_jY_i$ ($|j-i|\geqslant 2$), together with the relations $Y_i^2=0$. We describe an explicit presentation for the cohomology ring $Z\cong\mathsf{Ext}^*_{\mathcal{N}S_n}(\mathbb{Z},\mathbb{Z})$, with $n-i$ new generators in degree $i$ for $0< i<n$, and all relations are quadratic. We show that this $\mathsf{Ext}$ ring is $\mathbb{Z}$-free, and that it is a semiprime Noetherian affine polynomial identity (PI) ring with Poincaré series $1/(1-t)^{n-1}$ and PI degree $2^{n-2}$. For any field of coefficients $\mathbf{k}$, we show that $\mathsf{Ext}^*_{\mathbf{k}\mathcal{N}S_n}(\mathbf{k},\mathbf{k})$ is $\mathbf{k}\otimes_{\mathbb{Z}} Z$. Similar results hold for other finite Coxeter types. In the final section we show that $Z$ is a Koszul algebra whose Koszul dual is a signed version of the nilcactus algebra, an algebra closely related to the cactus group. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2407_21175 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | The cohomology of the nilCoxeter algebra Benson, David J. Rings and Algebras K-Theory and Homology 16E30, 16L60, 16R20, 20F55, 20J06 The nilCoxeter algebra $\mathcal{N}S_n$ of the symmetric group $S_n$ is the algebra over $\mathbb{Z}$ with generators $Y_i$ ($1\leqslant i\leqslant n-1$), satisfying the braid relations $Y_iY_{i+1}Y_i=Y_{i+1}Y_iY_{i+1}$, $Y_iY_j=Y_jY_i$ ($|j-i|\geqslant 2$), together with the relations $Y_i^2=0$. We describe an explicit presentation for the cohomology ring $Z\cong\mathsf{Ext}^*_{\mathcal{N}S_n}(\mathbb{Z},\mathbb{Z})$, with $n-i$ new generators in degree $i$ for $0< i<n$, and all relations are quadratic. We show that this $\mathsf{Ext}$ ring is $\mathbb{Z}$-free, and that it is a semiprime Noetherian affine polynomial identity (PI) ring with Poincaré series $1/(1-t)^{n-1}$ and PI degree $2^{n-2}$. For any field of coefficients $\mathbf{k}$, we show that $\mathsf{Ext}^*_{\mathbf{k}\mathcal{N}S_n}(\mathbf{k},\mathbf{k})$ is $\mathbf{k}\otimes_{\mathbb{Z}} Z$. Similar results hold for other finite Coxeter types. In the final section we show that $Z$ is a Koszul algebra whose Koszul dual is a signed version of the nilcactus algebra, an algebra closely related to the cactus group. |
| title | The cohomology of the nilCoxeter algebra |
| topic | Rings and Algebras K-Theory and Homology 16E30, 16L60, 16R20, 20F55, 20J06 |
| url | https://arxiv.org/abs/2407.21175 |