Large odd induced subgraphs via odd cuts

Abstract

Gallai proved that every graph can be partitioned into two sets, each inducing a subgraph with all degrees even. We show that if a graph admits a bipartition in which every vertex has an odd number of neighbors in the opposite part, then it can be partitioned into two sets, each inducing a subgraph with all degrees odd. Consequently, every $n$-vertex graph without isolated vertices has an induced subgraph with all degrees odd on at least $n/5$ vertices, substantially improving the previously known universal lower bound.