Using impact-factor representation, we consider the
lepton pair production by an incident
high-energy
photon in a strong electromagnetic
field of a nucleus. By summing leading terms of
perturbation series, we obtain a
simple formula for the amplitude, valid
to all orders in ${\cal O}(\alpha Z)$
and arbitrary field of the nucleus.
Using these results,
we derive, in a simple manner, the results for the lepton
pair production by a virtual incident photon
in a Coulomb field.
For real incident photon our results coincide
with the known ones.
Also, a particular example of a
non-Coulomb potential is discussed in some detail.
