2 Un en, (n+1)2 lim=lim n→ooln n>∞ n2 =mt -r收效四t子小 绝对收敛 n=i n=l 令 n n n u u 1 lim + →  lim → = n 1 2 ( 1) + + n e n n e n 2 2 1 1 lim       + = → n n n e 1 1 =  e 因此   = − 1 2 ( 1) n n n e n   =  − 1 2 ( 1) n n n e n 收敛, 绝对收敛