  A new method of hierarchical clustering of graph vertexes is suggested. In
the method, the graph partition is determined with an equivalence relation
satisfying a recursive definition stating that vertexes are equivalent if the
vertexes they point to (or vertexes pointing to them) are equivalent. Iterative
application of the partitioning yields a hierarchical clustering of graph
vertexes. The method is applied to the citation graph of hep-th. The outcome is
a two-level classification scheme for the subject field presented in hep-th,
and indexing of the papers from hep-th in this scheme. A number of tests show
that the classification obtained is adequate.
