The set of all rational Cauchy sequences. \[M:=\{(a_n)_{n\in\mathbb N}~|~(a_n)_{n\in\mathbb N}\text{ is a rational Cauchy sequence}\},\]
together with an addition operation defined for any rational Cauchy sequences \((a_n)_{n\in\mathbb N},(b_n)_{n\in\mathbb N}\in M\) by: \[\begin{array}{rcl} (a_n)_{n\in\mathbb N}+(b_n)_{n\in\mathbb N}&:=&(a_n+b_n)_{n\in\mathbb N}\\ \end{array}\]
builds an algebraic structure \((M, +)\), which is a commutative group.
Proofs: 1