(nextprime(3^n)-nextprime(2^n))/2.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =33a(5) =107a(6) =333a(7) =1036a(8) =3153a(9) =9583a(10) =29010a(11) =87557a(12) =263679a(13) =793061a(14) =2383280a(15) =7158069a(16) =21490605a(17) =64504548a(18) =193579176a(19) =580868607a(20) =1742867913a(21) =5229128045a(22) =15688432647a(23) =47067395121a(24) =141206379612a(25) =423627527495