The Copnumber of a Graph is Bounded by [3/2 genus (G)] + 3
Bernd S. W. Schröder · Birkhäuser Boston eBooks · 2001
We prove that the copnumber of a finite connected graph of genus g is bounded by [3/2 g ]+3. In particular this means that the copnumber of a toroidal graph is bounded by 4. We also sketch a proof that the copnumber of a graph of genus 2 is bounded by 5.