  In this paper we present a systematic analysis on the average interaction
between the last protons and neutrons in atomic nuclei, which can be extracted
from the double differences of nuclear binding energies. The empirical average
proton-neutron interaction $V_{pn}$ thus derived from experimental data can be
described in a very simple form as the interplay of the nuclear mean field and
the pairing interaction. It is found that the smooth behavior as well as the
local fluctuations of the $V_{pn}$ in even-even nuclei with $N\neq Z$ are
dominated by the contribution from the proton-neutron monopole interactions. A
strong additional contribution from the isoscalar monopole interaction and
isovector proton-neutron pairing interaction is seen in the $V_{pn}$ for
even-even $N=Z$ nuclei and for the adjacent odd-$A$ nuclei with one neutron or
proton being subtracted.
