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 A334069 #31 Feb 16 2025 08:34:00
%S A334069 0,0,0,1,2,7,14,34,71,152,325,669,1405,2866,5931,12139,24782,50444,
%T A334069 102458,207945,420511,850518,1716168,3460304,6968639,14022029,
%U A334069 28189833,56631732,113697179,228115641,457456902,916899721,1836996851,3678943569,7365141297,14740076678,29490954290
%N A334069 Number of numbers <= 2^n that are the product of exactly four primes, not necessarily distinct.
%H A334069 Robert G. Wilson v, <a href="/A334069/b334069.txt">Table of n, a(n) for n = 1..53</a>
%H A334069 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/AlmostPrime.html">Almost Prime</a>.
%H A334069 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime</a>.
%F A334069 a(n) = A082996(2^n).
%e A334069 a(6) = 7 because
%e A334069 16 = 2 * 2 * 2 * 2,
%e A334069 24 = 2 * 2 * 2 * 3,
%e A334069 36 = 2 * 2 * 3 * 3,
%e A334069 40 = 2 * 2 * 2 * 5,
%e A334069 54 = 2 * 3 * 3 * 3,
%e A334069 56 = 2 * 2 * 2 * 7, and
%e A334069 60 = 2 * 2 * 3 * 5
%e A334069 are the seven numbers less than 2^6 = 64 that are each the product of four primes.
%t A334069 FourAlmostPrimePi[n_] := Sum[ PrimePi[n/(Prime@i*Prime@j*Prime@k)] - k + 1, {i, PrimePi[n^(1/4)]}, {j, i, PrimePi[(n/Prime@i)^(1/3)]}, {k, j, PrimePi@Sqrt[n/(Prime@i*Prime@j)]}]; Array[FourAlmostPrimePi[2^#] &, 37]
%Y A334069 Cf. A007053, A014613, A082996, A125527, A127396, A126279.
%Y A334069 Partial sums of A120035.
%K A334069 nonn
%O A334069 1,5
%A A334069 _Robert G. Wilson v_, Apr 13 2020