第一種の合流型超幾何関数(クンマー)

1F1[a; b; z] = 1+Σ[k=1, ∞] {a(a+1)・・・・(a+k-1)/b(b+1)・・・・(b+k-1)} z^k/k!

1F1[-n; -2n; z] = {n!/(2n)!} Σ[k=0, n] {(2n-k)!/(n-k)!k!} z^k