A Multi-scale Subtraction Scheme and Partial Renormalization Group   Equations in the $O(N)$-symmetric $\phi^4$-theory
  To resum large logarithms in multi-scale problems a generalization of $\MS$ is introduced allowing for as many renormalization scales as there are generic scales in the problem. In the new \lq\lq minimal multi-scale subtraction scheme'' standard perturbative boundary conditions become applicable. However, the multi-loop beta functions depend on the various renormalization scale ratios and a large logarithms resummation has to be performed on them. Using these improved beta functions the \lq\lq partial'' renormalization group equations corresponding to the renormalization point independence of physical quantities allows one to resum the logarithms. As an application the leading and next-to-leading order two-scale analysis of the effective potential in the $O(N)$-symmetric $\phi^4$-theory is performed. This calculation indicates that there is no stable vacuum in the broken phase of the theory for $1<N\leq4$. 
C. Ford, C. Wiesendanger