The 4-string Braid group $B_4$ has property RD and exponential mesoscopic rank

Sylvain Barré, Mikael Pichot

We prove that the braid group $B_4$ on 4 strings, as well as its central quotient $B_4/<z>$, have the property RD of Haagerup-Jolissaint. It follows that the automorphism group $\…