用N-ε定义来做对任意ε>0,都存在N1使任意N>N1时│aN-a│<ε(N为偶数)存在N2使任意N>N2时│aN-a│<ε(N为奇数)取N0=max{N1,N2}则对任意N>N0,│aN-a│<ε,即{an}收敛于a