It is a well known fact that in R-n a subset of minimal perimeter L among all sets of a given volume is also a set of maximal volume among all sets of the same perimeter L. This is called the reciprocity principle for isoperimetric problems. The aim of this note is to prove this relation in the case where the class of admissible sets is restricted to the subsets of some subregion G subset of R-n. Furthermore, we give a characterization of those (unbounded) convex subsets of R-2 in which the isoperimetric problem has a solution. The perimeter that we consider is the one relative to R-n.
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