Solution (source code)

= Solution

The ring of <p-adic integer>[p-adic integers] is the inverse limit
$$
\boxed{\mathbb Z_p=\varprojlim_n\mathbb Z/p^n\mathbb Z.}
$$
Equivalently, each element has a unique convergent expansion $\sum_{j\geq0}b_jp^j$ with digits $0\leq b_j<p$. It is a complete discrete valuation ring with maximal ideal $(p)$ and residue field $\mathbb F_p$.