a(n) = (3^n-1)/2 if n odd, (3^n-1)/8 if n even.

Terms

a(0) =0a(1) =1a(2) =1a(3) =13a(4) =10a(5) =121a(6) =91a(7) =1093a(8) =820a(9) =9841a(10) =7381a(11) =88573a(12) =66430a(13) =797161a(14) =597871a(15) =7174453a(16) =5380840a(17) =64570081a(18) =48427561a(19) =581130733a(20) =435848050a(21) =5230176601a(22) =3922632451a(23) =47071589413a(24) =35303692060a(25) =423644304721a(26) =317733228541