A nonlocal fractional Helmholtz equation
  In this paper we study some boundary value problems for a fractional analogue of second order elliptic equation with an involution perturbation in a rectangular domain. Theorems on existence and uniqueness of a solution of the considered problems are proved by spectral method. 
Mokhtar Kirane, Batirkhan K. Turmetov, Berikbol T. Torebek
math.AP