  In the hard pomeron theory with the number of colours Nc in the infinity
limit the diffractive amplitude obtained in [3] is compared with the results
found for Nc=3 in [1] and in the dipole approach in [5]. It is shown that the
double pomeron exchange contribution can be substituted by an equivalent triple
pomeron interaction term. After such a substitution the triple pomeron vertices
in [1,3,5] essentially coincide. It is demonstrated that, in any form, the
triple pomeron vertex is conformal invariant. It is also shown that higher
order densities in the dipole approach do not involve 1 to k pomeron verteces
with k>2 but are rather given by a set of pomeron fan diagrams with only a
triple pomeron coupling.
