  The nuclear symmetry energy calculated in the RPA from the pairing plus
symmetry force Hamiltonian with equidistant single-nucleon levels is for mass
number A=48 approximately proportional to T(T+1.025), where T is the isospin
quantum number. An isovector character of the pair field assumed to produce the
observed odd-even mass staggering is essential for this result. The RPA
contribution to the symmetry energy cannot be simulated by adding to the
Hartree-Fock-Bogolyubov energy a term proportional to the isospin variance in
the Bogolyubov quasiparticle vacuum, and there are significant corrections to
the approximation which consist in adding half the isocranking angular
velocity. The present calculation employs a smaller single-nucleon level
spacing than used in a previous investigation of the model.
