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

Terms

a(0) =3a(1) =18a(2) =80a(3) =316a(4) =1124a(5) =3976a(6) =13770a(7) =46398a(8) =155174a(9) =512700a(10) =1679218a(11) =5454768a(12) =17617664a(13) =56578816a(14) =180896064a(15) =576041630a(16) =1827788446a(17) =5781493590a(18) =18236550884a(19) =57379347420a(20) =180130119286a(21) =564319058202