This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A120050 #9 Mar 18 2025 12:11:07
%S A120050 0,0,0,2,47,671,8491,101787,1180751,13377156,148930536,1636170477,
%T A120050 17787688377,191742524399,2052389350029,21838745177567,
%U A120050 231206458686127,2437121982958248,25591920108631224,267840642082525459
%N A120050 Number of 9-almost primes less than or equal to 10^n.
%e A120050 There are 2 nine-almost primes up to 1000: 512 & 768.
%t A120050 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 A120050 Table[AlmostPrimePi[9, 10^n], {n, 12}]
%Y A120050 Cf. A066265, A014613, A116382.
%Y A120050 Cf. A006880, A036352, A109251, A114106, A114453, A120047, A120048, A120049, A120050, A120051, A120052, A120053, A116430.
%K A120050 nonn
%O A120050 0,4
%A A120050 _Robert G. Wilson v_, Feb 07 2006
%E A120050 a(13) and a(14) from _Robert G. Wilson v_, Jan 07 2007
%E A120050 a(15)-a(19) from _Henri Lifchitz_, Mar 18 2025