设X=k时概率最大
P(X=k)/P(X=k+1)=[λ^k*e^(-λ)/k!]/[λ^(k+1)*e^(-λ)/(k+1)!]
=(k+1)/λ>=1
即k>=λ-1
P(X=k)/P(X=k-1)=[λ^k*e^(-λ)/k!]/[λ^(k-1)*e^(-λ)/(k-1)!]
=λ/k>=1
即k<=λ
故当λ为整数时,k=λ或λ-1时,概率最大
当λ不为整数时,k=[λ]时,概率最大