Number of primes < 4^n.

Terms

a(0) =0a(1) =2a(2) =6a(3) =18a(4) =54a(5) =172a(6) =564a(7) =1900a(8) =6542a(9) =23000a(10) =82025a(11) =295947a(12) =1077871a(13) =3957809a(14) =14630843a(15) =54400028a(16) =203280221a(17) =762939111a(18) =2874398515a(19) =10866266172a(20) =41203088796a(21) =156661034233a(22) =597116381732