a(n) = PrimePi(4^n) - PrimePi(2^n).

Terms

a(0) =0a(1) =1a(2) =4a(3) =14a(4) =48a(5) =161a(6) =546a(7) =1869a(8) =6488a(9) =22903a(10) =81853a(11) =295638a(12) =1077307a(13) =3956781a(14) =14628943a(15) =54396516a(16) =203273679a(17) =762926860a(18) =2874375515a(19) =10866222782a(20) =41203006771a(21) =156660878622a(22) =597116085785