a(n) = Sum_{d|n} (n/d) * 3^(n-d).

Terms

a(0) =1a(1) =7a(2) =28a(3) =127a(4) =406a(5) =1756a(6) =5104a(7) =20575a(8) =61237a(9) =230122a(10) =649540a(11) =2579932a(12) =6908734a(13) =26044984a(14) =74578888a(15) =269985151a(16) =731794258a(17) =2799670555a(18) =7360989292a(19) =27392181562a(20) =75948764752a(21) =268482753172a(22) =721764371008