訂正
F(x,y,f(x,y)) = Σ[k=1, l]a(k)*G(k, x) + Σ[k=1, m]b(k)*H(k, y) + Σ[k=1, n]c(k)*I(k, x, y) + Σ[k=1, o]d(k)
追加
d(k) = 0, (k>=1, k<=o)