a(n) = number of primes < n^4.

Terms

a(0) =0a(1) =0a(2) =6a(3) =22a(4) =54a(5) =114a(6) =210a(7) =357a(8) =564a(9) =847a(10) =1229a(11) =1715a(12) =2334a(13) =3107a(14) =4052a(15) =5191a(16) =6542a(17) =8152a(18) =10022a(19) =12187a(20) =14683a(21) =17531a(22) =20768a(23) =24421a(24) =28546a(25) =33118a(26) =38236a(27) =43934a(28) =50203a(29) =57097