A194895 - OEIS

%I #12 Jul 06 2012 03:14:38

%S 0,-1,9,131,1396,13786,131537,1239745,11665803,109941002,1038723932,

%T 9841935529,93518105160,891022511814,8510960557278,81485246876212,

%U 781818951793344

%N Difference between the number of semiprimes less than 10^n and the number of primes less than 10^n

%C A066265 minus A006880

%e There are 2625 semiprimes less than 10^4 and there are 1229 primes less than 10^4, so a(4) = 2625 - 1229 = 1396.

%t SemiPrimePi[n_] := Sum[ PrimePi[n/Prime@ i] - i + 1, {i, PrimePi@ Sqrt@ n}]; f[n_] := SemiPrimePi[10^n - 1] - PrimePi[10^n - 1]; Array[f, 15, 0] (* _Robert G. Wilson v_, Jul 05 2012 *)

%Y Cf. A006880, A066265

%K sign

%O 0,3

%A _Harvey P. Dale_, Sep 04 2011