p-adic numbers
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If X is a nonempty set, a distance, or metric, on X is a function d from pairs of elements (x, y) of X to the nonnegative real numbers such that
$$ \begin{gathered} d(x,\,y) = 0\,if\,and\,only\,if\,x = y. \hfill \\ \hfill \\ \end{gathered} $$
(1)
$$ d(x,\,y) = d(y,\,x). $$
(2)
$$ d(x,\,y)\, \leqslant \,d(x,\,z) + d(z,\,y)\,for\,all\,z\, \in \,X. $$
(3)
A set X together with a metrid d is called a metric space. The same set X can give rise to many different metric spaces (X, d), as we’ll soon see.