  It was argued recently that the holographic higher spin theory features
non-local interactions. We further elaborate on these results using the Mellin
representation. The main difficulty previously encountered on this way is that
the Mellin amplitude for the free theory correlator is ill-defined. To resolve
this problem, instead of literally applying the standard definition, we propose
to define this amplitude by linearity using decompositions, where each term has
the associated Mellin amplitude well-defined. Up to a sign, the resulting
amplitude is equal to the Mellin amplitude for the singular part of the quartic
vertex in the bulk theory and, hence, can be used to analyze bulk locality.
From this analysis we find that the scalar quartic self-interaction vertex in
the holographic higher spin theory has a singularity of a special form, which
can be distinguished from generic bulk exchanges. We briefly discuss the
physical interpretation of such singularities and their relation to the Noether
procedure.
