  A nonperturbative approach to anisotropic superconductivity is developed
based on the idea of two-particle self-consistent (TPSC) theory by Vilk and
Tremblay. A sum-rule which the momentum-dependent pairing susceptibility
satisfies is derived. An effective pairing interaction between quasiparticles
is determined so that the susceptibility should fulfill this exact sum-rule, in
which fluctuations belonging to different symmetries couple at finite momentum.
It is demonstrated that the mode coupling between d-wave and s-wave pairing
fluctuations leads to suppression of the d-wave fluctuation near the Mott
insulator.
