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Let $X$ a set $\sigma$ to $X^2$ into $[0,\infty)$ a function such as:
1)$\sigma(x,y)=0$ implies $x=y$
2)$\sigma(x,y)=\sigma(y,x)$
3)$\sigma(x,z)\leq \sigma(x,y)+\sigma(y,z)$.
we define $B(x,\varepsilon)=\{y,|\sigma(x,y)-\sigma(x,x)|<\varepsilon\}$.
$\tau_\sigma$ has as base the familly of op...