  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.
