CRITICAL DISORDERED-SYSTEMS WITH CONSTRAINTS AND THE INEQUALITY NU-GREATER-THAN-2 D/

The renormalization group approach is used to study the effects of a ' 'canonical'' constraint (e.g., a fixed number of occupied bonds) on cr itical quenched disordered systems. The constraint is found to be alwa ys irrelevant, even near the ''random'' fixed point. This proves that alpha < 0, or that nu > 2/d. ''Canonical'' and ''grand canonical'' ave rages thus belong to the same universality class. Related predictions concerning the universality of non-self-averaging distributions are te sted by Monte Carlo simulations of the site-diluted Ising model on the cubic lattice. In this case, the approach to the asymptotic distribut ion for ''canonical'' averaging is slow, resulting in effectively smal ler fluctuations. [S0031-9007(98)06581-8].