另一种解决方法是直接计算积分。
≥nfn(0)fn(x)=∫1x∫1x1∫1x2...∫1xn−2∫1xn−1dxndxn−1...dx2dx1
fn(0)
fn(x)fn(x)=∑nt=0(−x)tt!(n−t)!
n=1f1(x)=∑1t=0(−x)tt!(n−t)!=1−x=∫1xdx1
n=kfn(x)=∑kt=0(−x)tt!(k−t)! , for k≥1
n=k+1
fn(x)=fk+1(x)=∫1xfk(x∗)dx∗
=∫1x∑kt=0(−x∗)tt!(k−t)!dx∗
=∑kt=0−(−x∗)t+1t!(k−t)!×(t+1)∣∣∣1x=∑kt=0−(−x∗)t+1(t+1)!(k−t)!∣∣∣1x
=∑k+1t=1−(−x∗)tt!(k−t+1)!∣∣∣1x
=∑k+1t=1(−1)t+1t!(k−t+1)!+∑k+1t=1(−x)tt!(k−t+1)!
=∑k+1t=1(−1)t+1Ck+1t(k+1)!+∑k+1t=1(−x)tt!(k−t+1)!
=1(k+1)!+∑k+1t=0(−1)t+1Ck+1t(k+1)!+∑k+1t=1(−x)tt!(k−t+1)!
=1(k+1)!−(1−1)k+1(k+1)!+∑k+1t=1(−x)tt!(k−t+1)!
=∑k+1t=0(−x)tt!(k−t+1)!
通过数学归纳法,该假设成立。
fn(0)=1n!
E(length)=∑∞n=1Pr(length≥n)=∑∞n=11n!=e−1