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

Terms

a(0) =0a(1) =1a(2) =7a(3) =31a(4) =115a(5) =391a(6) =1267a(7) =3991a(8) =12355a(9) =37831a(10) =115027a(11) =348151a(12) =1050595a(13) =3164071a(14) =9516787a(15) =28599511a(16) =85896835a(17) =257887111a(18) =774054547a(19) =2322950071a(20) =6970423075a(21) =20914414951a(22) =62749536307a(23) =188261191831a(24) =564808741315