Kauffman monoids $\mathcal{ K}_n$ and Jones monoids $\mathcal{ J}_n$, $n=2,3,\dots$, are two families of monoids relevant in knot theory. We prove a somewhat counterintuitive result that the Kauffman monoids $\mathcal{ K}_3$ and $\mathcal{ K}_4$ satisfy exactly the same identities. This leads to a polynomial time algorithm to check whether a given identity holds in $\mathcal{ K}_4$. As a byproduct, we also find a polynomial time algorithm for checking identities in the Jones monoid $\mathcal{ J}_4$.
Categories: math.GR cs.CC
