a(n) = floor(sqrt(pi(2^n))).

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =2a(5) =3a(6) =4a(7) =5a(8) =7a(9) =9a(10) =13a(11) =17a(12) =23a(13) =32a(14) =43a(15) =59a(16) =80a(17) =110a(18) =151a(19) =208a(20) =286a(21) =394a(22) =544a(23) =751a(24) =1038a(25) =1436a(26) =1989a(27) =2757a(28) =3825a(29) =5309