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.