a(n) = 3*(2*n - 1)*( 3*(2*n - 1)^3 + 1) / 2.

Terms

a(0) =6a(1) =369a(2) =2820a(3) =10815a(4) =29538a(5) =65901a(6) =128544a(7) =227835a(8) =375870a(9) =586473a(10) =875196a(11) =1259319a(12) =1757850a(13) =2391525a(14) =3182808a(15) =4155891a(16) =5336694a(17) =6752865a(18) =8433780a(19) =10410543a(20) =12715986a(21) =15384669a(22) =18452880a(23) =21958635a(24) =25941678a(25) =30443481a(26) =35507244a(27) =41177895a(28) =47502090a(29) =54528213