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 A205726 #22 Jul 23 2024 18:49:54
%S A205726 0,1,3,6,9,13,17,22,26,34,40,48,56,62,75,82,90,103,114,126,135,149,
%T A205726 164,179,190,202,220,236,253,270,289,304,320,340,360,381,404,425,443,
%U A205726 462,484,508,533,556,581,604,634,655,678,709,738,761,783,813,846,881
%N A205726 Number of semiprimes <= n^2.
%C A205726 See A205727 and A205728 for related sequences and relationship to Goldbach conjecture.
%H A205726 Harvey P. Dale, <a href="/A205726/b205726.txt">Table of n, a(n) for n = 1..1000</a>
%F A205726 a(n) = A072000(A000290(n)). - _Michel Marcus_, Sep 02 2013
%t A205726 SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; nn = 100; t = Select[Range[nn^2], SemiPrimeQ]; Table[Length[Select[t, # <= n^2 &]], {n, nn}] (* _T. D. Noe_, Jan 30 2012 *)
%t A205726 Module[{nn=60,sp},sp=Accumulate[Table[If[PrimeOmega[n]==2,1,0],{n,nn^2}]];Table[sp[[i^2]],{i,nn}]] (* _Harvey P. Dale_, May 29 2014 *)
%o A205726 (Python)
%o A205726 from sympy import prime, primepi
%o A205726 def A205726(n): return int(sum(primepi(n**2//prime(k))-k+1 for k in range(1,primepi(n)+1))) # _Chai Wah Wu_, Jul 23 2024
%Y A205726 Cf. A001358, A038108, A205727, A205728.
%K A205726 nonn
%O A205726 1,3
%A A205726 _Keith Backman_, Jan 30 2012