On a Duality between Metrics and $\Sigma$-Proximities
  : In studies of discrete structures, functions are frequently used that express proximity, but are not metrics. We consider a class of such functions that is characterized by a normalization condition and an inequality that plays the same role as the triangle inequality does for metrics. We show that the introduced functions, named $\Sigma$-proximities, are in a definite sense dual to metrics: there exists a natural one-to-one correspondence between metrics and $\Sigma$-proximities defined on the same finite set; in contrast to metrics, $\Sigma$-proximities measure {\it comparative} proximity; the closer the objects, the greater the $\Sigma$-proximity; diagonal entries of the $\Sigma$-proximity matrix characterize the ``centrality'' of elements. The results are extended to arbitrary infinite sets. 
P. Yu. Chebotarev and E. V. Shamis