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%I #6 Jul 06 2012 11:48:16
%S 0,0,0,2,47,671,8491,101787,1180751,13377156,148930536,1636170477,
%T 17787688377,191742524399,2052389350029
%N Number of 9-almost primes less than or equal to 10^n.
%e There are 2 nine-almost primes up to 1000: 512 & 768.
%t AlmostPrimePi[k_Integer, n_] := Module[{a, i}, a[0] = 1; If[k == 1, PrimePi[n], Sum[PrimePi[n/Times @@ Prime[Array[a, k - 1]]] - a[k - 1] + 1, Evaluate[ Sequence @@ Table[{a[i], a[i - 1], PrimePi[(n/Times @@ Prime[Array[a, i - 1]])^(1/(k - i + 1))]}, {i, k - 1}]]]]]; (* _Eric W. Weisstein_, Feb 07 2006 *)
%t Table[AlmostPrimePi[9, 10^n], {n, 12}]
%Y Cf. A066265, A014613, A116382.
%Y Cf. A006880, A036352, A109251, A114106, A114453, A120047, A120048, A120049, A120050, A120051, A120052, A120053, A116430.
%K nonn
%O 0,4
%A _Robert G. Wilson v_, Feb 07 2006
%E a(13) and a(14) from _Robert G. Wilson v_, Jan 07 2007