On Relative Central Extensions and Covering Pairs
  Let $(G,N)$ be a pair of groups. In this article, first we construct a relative central extension for the pair $(G,N)$ such that special types of covering pair of $(G,N)$ are homomorphic image of it. Second, we show that every perfect pair admits at least one covering pair. Finally, among extending some properties of perfect groups to perfect pairs, we characterize covering pairs of a perfect pair $(G,N)$ under some extra assumptions. 
Azam Pourmirzaei, Mitra Hassanzadeh and Behrooz Mashayekhy
math.GR