A topological characterisation of the Kashiwara-Vergne groups

In 2017 Bar-Natan and the first author showed that solutions to the Kashiwara--Vergne equations are in bijection with certain knot invariants: homomorphic expansions of welded foams. Welded foams are a class of knotted tubes in $mathbb{R}^4$, which can be finitely presented algebraically as a circuit algebra, or, equivalently, a wheeled prop. In this paper we describe the Kashiwara-Vergne groups $mathsf{KV}$ and $mathsf{KRV}$ -- the symmetry groups of Kashiwara-Vergne solutions -- as automorphisms of the completed circuit algebras of welded foams, and their associated graded circuit algebra of arrow diagrams, respectively. Finally, we provide a description of the graded Grothendieck-Teichmuller group $mathsf{GRT}_1$ as automorphisms of arrow diagrams.