Word maps and Waring type problems

Waring's classical problem deals with expressing every natural number as a sum of g(k) k-th powers. Recently there has been considerable interest in similar questions for nonabelian groups, and simple groups in particular. Here the k-th power word is replaced by an arbitrary nontrivial group word w, and the goal is to express group elements as short products of values of w. We give a best possible and somewhat surprising solution for this Waring type problem for various finite simple groups, showing that a product of length two suffices to express all elements. We also show that the set of values of w is very large, improving various results obtained previously. Along the way we also obtain new results of independent interest on character values and class squares in symmetric groups. Our methods involve algebraic geometry, representation theory, probabilistic arguments, as well as three prime theorems from additive number theory (approximating Goldbach's Conjecture).