G.f. = Phi^3*F^3, where Phi = g.f. for A028930, F = g.f. for A028959.

Terms

a(0) =1a(1) =6a(2) =18a(3) =50a(4) =132a(5) =276a(6) =530a(7) =1008a(8) =1710a(9) =2760a(10) =4464a(11) =6828a(12) =10268a(13) =15246a(14) =21828a(15) =30820a(16) =42750a(17) =57696a(18) =77032a(19) =101916a(20) =132564a(21) =169736a(22) =216816a(23) =269286a(24) =338490a(25) =412386a(26) =507354a(27) =605278a(28) =739308a(29) =865314