Nowhere-zero flows in signed series-parallel graphs
  Bouchet conjectured in 1983 that each signed graph that admits a nowhere-zero flow has a nowhere-zero 6-flow. We prove that the conjecture is true for all signed series-parallel graphs. Unlike the unsigned case, the restriction to series-parallel graphs is nontrivial; in fact, the result is tight for infinitely many graphs. 
Tom\'a\v{s} Kaiser and Edita Rollov\'a
math.CO