若級數Un收斂於s則級數(un+un+1)收斂於

若級數Un收斂於s則級數(un+un+1)收斂於


∑(n>=1)u(n)= s,
可得
∑(n>=1)[u(n)+u(n+1)]
=∑(n>=1)u(n)+∑(n>=1)u(n+1)
= 2s-u(1).