Series for the Wilson functions of an ‘‘environmentally friendly’’ renormalization group are computed to two loops, for an O(N) vector model, in terms of the ‘‘floating coupling,’’ and resummed by the Padé method to yield crossover exponents for finite size and quantum systems. The resulting effective exponents obey all scaling laws, including hyperscaling in terms of an effective dimensionality, ${\mathit{d}}_{\mathrm{eff}}$=4-${\gamma }_{\lambda }$, which represents the crossover in the leading irrelevant operator, and are in excellent agreement with known results.

• Received 30 August 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.506