Topological Charge of ADHM Instanton on R^2_{NC}*R^2
  We have calculated the topological charge of U(N) instantons on non-degenerate noncommutative space time to be exactly the instanton number k in a previous paper [Mod.Phys.Lett. A18 1691]. This paper, which deals with the degenerate R^2_{NC}*R^2 case, is the continuation of that one. We find that the same conclusion holds in this case, thus complete the answer to the problem of topological charge of noncommutative U(N) instantons. 
Yu Tian