a(n) is the number of prime powers <= 3^n.

Terms

a(0) =1a(1) =3a(2) =8a(3) =16a(4) =33a(5) =69a(6) =153a(7) =360a(8) =894a(9) =2294a(10) =6060a(11) =16229a(12) =44126a(13) =121021a(14) =334771a(15) =931905a(16) =2608160a(17) =7333717a(18) =20703268a(19) =58652234a(20) =166684333a(21) =475034096a(22) =1357217584a(23) =3886575564a(24) =11152863757a(25) =32065004164a(26) =92349161271a(27) =266398439961a(28) =769616851854