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Preprints, Working Papers, ... Year : 2022

Random walks and the symplectic representation of the braid group

Abstract

We consider the symplectic representation $\rho_n$ of a braid group $B(n)$ in $Sp(2l,\mathbb{Z})$, for $l=\Big[\dfrac{n-1}{2}\Big]$. If $P$ is a $4l^2$ polynomial on the coefficients of the matrices in $Sp(2l,\mathbb{Z})$, we show that the set $\{\beta\in B(n): P(\rho_n(\beta))=0\}$ is transient for non degenerate random walks on $B(n)$. If $n$ is odd, we derive that the $n$-braids $\beta$ verifying $|\Delta_{\hat{\beta}}(-1)|\leq C$ for some constant $C$ form a transient set: here $\Delta_{\hat{\beta}}$ denotes the Alexander polynomial of the closure of $\beta$. We also derive that for a random $3$-braid, the quasipositive links $(\beta\sigma_i\beta^{-1}\sigma_j)^p$ have zero signature for every integer $p$ and $1\leq i,j\leq 2$. \\ As an example of such braids, we investigate the signature of the Lissajous toric knots with $3$ strands.
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Dates and versions

hal-03864935 , version 1 (28-11-2022)

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Marc Soret, Marina Ville. Random walks and the symplectic representation of the braid group. 2022. ⟨hal-03864935⟩
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