On the basis property of the root function systems of regular boundary   value problems for the Sturm-Liouville operator
  We consider the nonselfadjoint Sturm-Liouville operator Lu=u''-q(x)u with regular but not strongly regular boundary conditions, where q(x)is an arbitrary complex-valued summable function. We examine the basis property for the root function system of the mentioned operator. 
Alexander Makin