0563132人目の素数さん
2019/06/16(日) 23:56:19.17ID:HwIhVDIU(1+z)e^(-z) -1 = -zz + (1+z)g(z)
= -zz + (1+z)Σ[k=2,∞] (1/k!)(-z)^k
= -Σ[k=2,∞] {1/(k-1)! - 1/k!}(-z)^k,
(左辺) ≦ Σ[k=2,∞] {1/(k-1)! - 1/k!}|z|^k
≦ |z|^2・Σ[k=2,∞] {1/(k-1)! - 1/k!} (|z|≦1)
= |z|^2,
等号成立は z=-1 のとき。