棒極了,續個貂.P(K)=(K/N){1/K + 1/(K+1) + ... + 1/(N-1)}=

(K/N){1/K　+　1/(K+1)　+　...　+　1/(N-1)+1+1/2+1/3 +....+1/(K-1)-1-1/2-1/3-...1/(K-1)}=K/N){1+1/2+1/3+...+1/(N-1)-[1+1/2+1/3+...1/(K-1)]}
當K,N很大時與K/N{LN(N)-LN(K)}=(K/N)LN(N/K)等價...,XLN(1/X)=-XLNX,求導,-1+LNX=0,X=1/E達極值.  LN底為E
K,N不大時,K/N在1/E附近,誤差有一些,如N=5,K=2.確值P=13/30=.4333...,.0.4LN(2.5)=.3665,