a(n) = prime(3^n - 1).

Terms

a(0) =3a(1) =19a(2) =101a(3) =409a(4) =1531a(5) =5507a(6) =19273a(7) =65677a(8) =220859a(9) =733559a(10) =2412763a(11) =7867543a(12) =25485179a(13) =82064011a(14) =262960069a(15) =839001707a(16) =2666790157a(17) =8448283721a(18) =26684834629a(19) =84064182049a(20) =264194301341a(21) =828513359543