Given a set of points $p_1, p_2, ..., p_n$ and a collection of sets from $n$ points $S_1, S_2,...,S_m$. The pairwise distance between any three points $(p_1, p_2, p_3)$ satisfies triangle inequality $d(p_1,p_2) + d(p_2, p_3) \geq d(p_1, p_3)$. How to define the distance between sets such that the pairwise distance between any three sets $S_1, S_2, S_3$ satisfies triangle inequality $d(S_1,S_2) + d(S_2, S_3) \geq d(S_1, S_3)$. For example, when every set is $\textbf{pairwise disjoint}$ and has the $\textbf{same cardinality}$, we can define the distance between any two set to be the sum of the distances between elements of the two sets $$d(S_1, S_2) = \sum_{x \in S_1, y \in S_2} d(x,y)$$. we can show that the triangle inequality is satisfied for any three sets.
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