a(n) = 3^n - prevprime(3^n), where prevprime(x) is the largest prime < x.

Terms

a(0) =1a(1) =2a(2) =4a(3) =2a(4) =2a(5) =2a(6) =8a(7) =8a(8) =2a(9) =20a(10) =16a(11) =58a(12) =22a(13) =8a(14) =16a(15) =98a(16) =10a(17) =10a(18) =14a(19) =8a(20) =4a(21) =2a(22) =20a(23) =28a(24) =20a(25) =20a(26) =26a(27) =22a(28) =14a(29) =26