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 A358078 #18 Apr 02 2023 09:02:13
%S A358078 0,0,0,1,4,7,18,37,76,149,293,575,1106,2162,4161,8068,15604,30181,
%T A358078 58449,113179,219587,425951,827393,1608250,3128647,6090677,11867571,
%U A358078 23139485,45148817,88155104,172231561,336713062,658655523,1289140675,2524520079,4946303842
%N A358078 a(n) is the number of squarefree semiprimes <= 2^n.
%C A358078 I.e., number of numbers <= 2^n that are the product of two distinct primes.
%e A358078 a(5) = 7 because there are 7 squarefree semiprimes <= 2^5 = 32:
%e A358078 2*3 = 6, 2*5 = 10, 2*7 = 14, 2*11 = 22, 2*13 = 26,
%e A358078 3*5 = 15, 3*7 = 21.
%e A358078 a(6) = 18 because there are 18 squarefree semiprimes <= 2^6 = 64:
%e A358078 2*{3,5,7,11,13,17,19,23,29,31} = {6,10,14,22,26,34,38,46,58,62},
%e A358078 3*{5,7,11,13,17,19} = {15,21,33,39,51,57},
%e A358078 5*{7,11} = {35,55}.
%t A358078 Reap[Do[Do[If[And[PrimeOmega[#] == 2, SquareFreeQ[#]], c++] &[k], {k, 2^(n - 1) + 1, 2^n}]; Sow[c], {n, 0, 16}]][[-1, -1]] (* _Michael De Vlieger_, Oct 30 2022 *)
%t A358078 Module[{nn=22,sfsp},sfsps=Table[If[PrimeOmega[n]==2&&SquareFreeQ[n],1,0],{n,2^nn}];Table[ Total[ Take[sfsps,2^x]],{x,0,nn}]] (* _Harvey P. Dale_, Apr 02 2023 *)
%o A358078 (PARI) a(n) = sum(k=1, 2^n, bigomega(k)==2&&issquarefree(k)); \\ _Michel Marcus_, Oct 30 2022
%Y A358078 Cf. A006881, A007053, A036351.
%K A358078 nonn
%O A358078 0,5
%A A358078 _Jon E. Schoenfield_, Oct 29 2022