Chapter 3.2, Problem 28E

Reword Definition $3.6$ for a group with respect to addition.

Definition $3.6$

: Product Notation

Let $n$ be a positive integer, $n\ge 2$. For elements $a_1,a_2,\mathrm{...},a_n$ in a group $G$, the expression $a_1{a}_2\mathrm{...}{a}_n$ is defined recursively by $a_1{a}_2\mathrm{...}{a}_k{a}_{k+1}=(a_1{a}_2\mathrm{...}{a}_k)a_{k+1}$ for $k\ge 1$.