Number of primes <= 7^n.

Terms

a(0) =0a(1) =4a(2) =15a(3) =68a(4) =357a(5) =1939a(6) =11098a(7) =65685a(8) =397764a(9) =2453911a(10) =15353323a(11) =97163605a(12) =620646217a(13) =3995149838a(14) =25885182840a(15) =168650876819