Title: Transfinite Computation and a problem of Richter

Abstract: We use a model of transfinite computation to give an alternative
proof of a theorem of Harrington and of S.Friedman on the re-occurrence
of theories
in the constructible hierarchy.

W. Richter asked whether the least pair of ordinals (zeta_n,Sigma_n)
such that the Sigma_n theories of L_{zeta_n} and L_{Sigma_n} are
identical, is also the least
pair such that L_Sigma_n has a Sigma_2 elementary substructure