A knot-theoretic approach to comparing the Grothendieck-Teichmüller and Kashiwara-Vergne groups

Homomorphic expansions are combinatorial invariants of knotted objects, which are universal in the sense that all finite-type (Vassiliev) invariants factor through them. Homomorphic expansions are also important as bridging objects between low-dimensional topology and quantum algebra. For example, homomorphic expansions of parenthesised braids are in one-to-one correspondence with Drinfel’d associators (Bar-Natan 1998), and homomorphic expansions of $w$-foams are in one-to-one correspondence with solutions to the Kashiwara-Vergne (KV) equations (Bar-Natan and the first author, 2017). The sets of Drinfel’d associators and KV solutions are both bi-torsors, with actions by the pro-unipotent Grothendieck-Teichmüller and Kashiwara-Vergne groups, respectively. The above correspondences are in fact maps of bi-torsors (Bar-Natan 1998, and the first and third authors with Halacheva 2022). There is a deep relationship between Drinfel’d associators and KV equations–discovered by Alekseev, Enriquez and Torossian in the 2010s–including an explicit formula constructing KV solutions in terms of associators, and an injective map $\rho :{\mathsf{G}\mathsf{R}\mathsf{T}}_1\to \mathsf{K}\mathsf{R}\mathsf{V}$. This paper is a topological/diagrammatic study of the image of the Grothendieck-Teichmüller groups in the Kashiwara-Vergne symmetry groups, using the fact that both parenthesised braids and $w$-foams admit respective finite presentations as an operad and as a tensor category (circuit algebra or prop).