On D = 5 super Yang-Mills theory and (2, 0) theory

We discuss how D = 5 maximally supersymmetric Yang-Mills theory (MSYM) might be used to study or even to define the (2, 0) theory in six dimensions. It is known that the compactification of (2, 0) theory on a circle leads to D = 5 MSYM. A variety of arguments suggest that the relation can be reversed, and that all of the degrees of freedom of (2, 0) theory are already present in D = 5 MSYM. If so, this relation should have consequences for D = 5 SYM perturbation theory. We explore whether it might imply all orders finiteness, or else an unusual relation between the cutoff and the gauge coupling. S-duality of the reduction to D = 4 may provide nonperturbative constraints or tests of these options.