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arXiv 2107.12263math.AT
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模4辫群的上同调

The Cohomology of the Mod 4 Braid Group

Trevor Nakamura

AI总结:

本文构造了分类模4辫群扩张的2-上循环,证明其为已知上循环的模2约化,并给出扩张表示及level 4同余子群的正规生成集。

摘要:

模4辫群 $\mathcal{Z}_{n}$ 被定义为辫群除以由所有元素的平方生成的纯辫群子群的商群。Kordek 和 Margalit 证明了 $\mathcal{Z}_{n}$ 是对称群通过 $\mathbb{Z}_{2}^{\binom{n}{2}}$ 的扩张。对于 $n\geq 1$,我们在对称群的群上同调中构造了一个具有扭曲系数的 2-上循环,该上循环分类了 $\mathcal{Z}_{n}$。我们证明这个上循环是对应于对称群通过纯辫群的交换化扩张的 2-上循环的模 2 约化。我们还构造了对应于这第二个扩张的 2-上循环,并证明它在对称群的上同调中代表一个二阶元素。此外,我们给出了这两个扩张的表示,以及辫群的 level 4 同余子群的一个正规生成集。

英文摘要:

The mod 4 braid group, $\mathcal{Z}_{n}$, is defined to be the quotient of the braid group by the subgroup of the pure braid group generated by squares of all elements. Kordek and Margalit proved $\mathcal{Z}_{n}$ is an extension of the symmetric group by $\mathbb{Z}_{2}^{\binom{n}{2}}$. For $n\geq 1$, we construct a 2-cocycle in the group cohomology of the symmetric group with twisted coefficients classifying $\mathcal{Z}_{n}$. We show this cocycle is the$\mod2$ reduction of the 2-cocycle corresponding to the extension of the symmetric group by the abelianization of the pure braid group. We also construct the 2-cocycle corresponding to this second extension and show it represents an order two element in the cohomology of the symmetric group. Furthermore, we give presentations for both extensions and a normal generating set for the level 4 congruence subgroup of the braid group.

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