Let f be a non-commutative polynomial such that f=0 if we assume that the variables in f commute. Let Q(f) be the smallest k such that there exist polynomials g_1,g_1', g_2, g_2',\dots, g_k, g_k' with

where [g,h]=gh-hg. Then Q(f)\leq {n\choose 2}, where ... more >>>