a(n) = 4a(n-1) + 3n with a(0) = 0.

Terms

a(0) =0a(1) =3a(2) =18a(3) =81a(4) =336a(5) =1359a(6) =5454a(7) =21837a(8) =87372a(9) =349515a(10) =1398090a(11) =5592393a(12) =22369608a(13) =89478471a(14) =357913926a(15) =1431655749a(16) =5726623044a(17) =22906492227a(18) =91625968962a(19) =366503875905