7-adic integer (${\mathbb{Z}}_7$)

A 7-adic integer is an element of the p-adic integers for $p=7$, equivalently a compatible sequence of residues modulo $7^r$. Its p-adic digits lie in $\{0,1,\dots ,6\}$. For example, $10=3+1\cdot 7$ has low-to-high digits $3,1,0,\dots$.