Definition:Series/Tail

Definition
Let $\struct {S, \circ}$ be a semigroup.

Let $\sequence {a_n}$ be a sequence in $S$. Let $N \in \N$.

The expression $\ds \sum_{n \mathop = N}^\infty a_n$ is known as a tail of the series $\ds \sum_{n \mathop = 1}^\infty a_n$.