a(n) = 15*2^n + 6*4^n + 10*3^n + 3*5^n + 6^n + 21.

Terms

a(0) =56a(1) =126a(2) =378a(3) =1386a(4) =5778a(5) =26226a(6) =126378a(7) =636426a(8) =3314178a(9) =17714466a(10) =96660378a(11) =536249466a(12) =3015243378a(13) =17141522706a(14) =98333399178a(15) =568324150506