  We study global features of proton-neutron (p-n) interactions and symmetry
energy over a wide range of nuclei, using a schematic model interaction with
four forces proposed recently.
  Calculations are performed by the BCS approximation in N,Z=20-50 and
N,Z=50-82 regions. The experimental double differences of binding energies and
symmetry energy are reproduced quite well.
  It is shown that the isoscalar p-n interactions with all J are indispensable
for explaining the binding energies of not only
  $N\approx Z$ but also N>Z nuclei in the A=40-160 region.
