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 A080774 #28 Jun 15 2025 08:58:04
%S A080774 15,35,39,51,55,87,91,95,111,115,119,123,143,155,159,183,187,203,215,
%T A080774 219,235,247,259,267,287,291,295,299,303,319,323,327,335,339,355,371,
%U A080774 391,395,403,407,411,415,427,447,451,471,511,515,519,527,535,543,551
%N A080774 Numbers with two prime factors: (4*i+1)*(4*j+3).
%C A080774 Semiprimes of the form 4k+3. - _Giovanni Teofilatto_, Jun 15 2005
%C A080774 There are 971 semiprimes of the form 4k+3 below 10,000.
%H A080774 Amiram Eldar, <a href="/A080774/b080774.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe)
%H A080774 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime</a>.
%H A080774 Wikipedia, <a href="http://en.wikipedia.org/wiki/Semiprime">Semiprime</a>.
%e A080774 a(1) = 15 = 3*5 = (4*1+1)*(4*0+3).
%t A080774 Select[Range[15, 1000, 2], Last /@ FactorInteger[#] == {1, 1} && IntegerQ[(# - 3)/4] &] (* _Vladimir Joseph Stephan Orlovsky_, May 07 2011 *)
%t A080774 Select[4*Range[200]+3,PrimeOmega[#]==2&] (* _Harvey P. Dale_, Nov 02 2024 *)
%o A080774 (PARI) isok(k) = k % 4 == 3 && bigomega(k) == 2; \\ _Amiram Eldar_, Jun 15 2025
%Y A080774 Cf. A001358, A006881, A002144, A002145, A107978, A121387.
%K A080774 nonn
%O A080774 1,1
%A A080774 _Reinhard Zumkeller_, Mar 10 2003