Another question regarding the Yang-Mills Existence and Mass Gap problem (http://www.claymath.org/sites/default/files/yangmills.pdf). Does the problem require that the "construction" of a four dimensional quantum Yang-Mills be non-perturbative? I get the feeling that this problem is to make notions such as the renormalization group rigorous, and thus is perturbative, but isn't lattice gauge theory already mathematically well-defined? If so, why can this not be used as an approach to this problem? Essentially, which is the preferable approach as specified by the problem: perturbative or non-perturbative?