Welded extensions and ribbon restrictions of diagrammatical moves

Abstract

In this paper, we consider local moves on classical and welded diagrams of string links, and the notion of welded extension of a classical move. Such extensions being non-unique in general, the idea is to find a topological criterion which could isolate one extension from the others. To that end, we turn to the relation between welded string links and knotted surfaces in $\mathbb{R}^4$, and the ribbon sublclass of these surfaces. This provides the topological interpretation of classical local moves as surgeries on surfaces, and of welded local moves as surgeries on ribbon surfaces. Comparing these surgeries leads to the notion of ribbon residue of a classical local move, and we show that up to some broad conditions there can be at most one welded extension which is a ribbon residue. The existence of such an extension is not guaranteed however, and we provide a counterexample.