Correlations in orbital angular momentum of spatially entangled paired photons generated in parametric down-conversion

What are the orbital angular momentum correlations between spatially entangled photon pairs generated in spontaneous parametric down-conversion? We show that the answer to this question can be given in two alternative, although complementary, ways. The answer posed in this Brief Report explains satisfactorily the seemingly contradictory results obtained in different experiments, and theoretical approaches.