The limit laws for series are almost identical to the limit laws for sequences and for functions. If ∞∑n=1an=L and ∞∑n=1bn=M (i.e., the series converge), then
∞∑n=1(an+bn)=L+M
∞∑n=1(an−bn)=L−M
∞∑n=1(can)=cL
There are no limit laws for ∞∑n=1(anbn) or ∞∑n=1(anbn).
Notice that
If ∞∑n=1an converges, then limn→∞an=0.
Conversely, if limn→∞an is not zero (or does not exist), then ∞∑n=1an diverges. (This is also called the Divergence test.)