Split-Free Cable Expressions: Active Neighborhood Profiles and Linear Rank-Width

Abstract

We introduce split-free cable expressions and their sequential restriction. Live cables are vertex blocks that future operations cannot split. The main result identifies sequential split-free cable width, up to an additive two, with active neighborhood-width, the minimum number of distinct nonzero future-neighborhood classes across a linear layout. The lower bound extracts such a layout from every cable play and includes a parity argument for prefixes inside a birth group. The upper bound compiles any active-neighborhood layout into a singleton-birth play. This gives an exponential upper bound and a linear lower bound in terms of linear rank-width. We prove that the complement of the seven-vertex cycle has linear rank-width two and sequential cable width exactly eight. This disproves the proposed affine upper estimate already at rank two. We also prove an unbounded separation between branching expression width and sequential width on trees.