Multivariate Alexander quandles, II. The involutory medial quandle of a link (corrected) (1902.10603v9)

Abstract: Joyce showed that for a classical knot $K$, the involutory medial quandle $\text{IMQ}(K)$ is isomorphic to the core quandle of the homology group $H_1(X_2)$, where $X_2$ is the cyclic double cover of $\mathbb S ^3$, branched over $K$. It follows that $|\text{IMQ}(K)| = | \det K |$. In the present paper, the extension of Joyce's result to classical links is discussed. Among other things, we show that for a classical link $L$ of $\mu \geq 2$ components, the order of the involutory medial quandle is bounded as follows: [ \frac{\mu | \det L |}{2} \geq |\text{IMQ}(L)| \geq \frac{ \mu | \det L |} {2^{\mu -1}}. ] In particular, $\text{IMQ}(L)$ is infinite if and only if $\det L =0$. We also show that in general, $\text{IMQ}(L)$ is a strictly stronger invariant than $H_1(X_2)$. That is, if $L$ and $L'$ are links with $\text{IMQ}(L) \cong \text{IMQ}(L')$, then $H_1(X_2) \cong H_1(X'_2)$; but it is possible to have $H_1(X_2) \cong H_1(X'_2)$ and $\text{IMQ}(L) \not \cong \text{IMQ}(L')$. In fact, it is possible to have $X_2 \cong X'_2$ and $\text{IMQ}(L) \not \cong \text{IMQ}(L')$.