Chapter B, Problem 2CV

If $a_1,a_2,\mathrm{...},a_n,\mathrm{...}$ is some collection of numbers, the expression

${\displaystyle \underset{k=1}{\overset{\infty }{\sum }}{a}_k=a_1+a_2+\cdot \cdot \cdot }+a_n+\cdot \cdot \cdot$ is called $a\left(n\right)$ _____ _____.