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 A107978 #25 Feb 16 2025 08:32:57
%S A107978 9,21,33,49,57,69,77,93,121,129,133,141,161,177,201,209,213,217,237,
%T A107978 249,253,301,309,321,329,341,361,381,393,413,417,437,453,469,473,489,
%U A107978 497,501,517,529,537,553,573,581,589,597,633,649,669,681,713,717,721,737
%N A107978 Products of two primes of the form 4n+3 (A002145).
%C A107978 Every odd semiprime must be in one of three disjoint sets: the product of two primes of the form 4n+1 (A121387), the product of two primes of the form 4n+3 (A107978), or the product of a prime of the form 4n+1 and a prime of the form 4n+3 (A080774).
%H A107978 T. D. Noe, <a href="/A107978/b107978.txt">Table of n, a(n) for n=1..1000</a>
%H A107978 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime.</a>
%F A107978 {a(n)} = {p*q: p and q both elements of A002145}.
%t A107978 p = Select[ Prime@ Range@ 60, Mod[ #, 4] == 3 &]; Take[ Sort@ Flatten@ Table[ p[[i]] p[[j]], {j, 30}, {i, j}], 54] (* or *)
%t A107978 fQ[n_] := Block[{fi = FactorInteger@ n}, Plus @@ Last /@ fi == 2 && Union@ Mod[ First /@ fi, 4] == {3}]; Select[ Range@ 748, fQ@# &] (* _Robert G. Wilson v_, May 20 2010 *)
%Y A107978 Cf. A001358, A002145, A080774, A121387.
%Y A107978 Union of A131574 and A080109.
%Y A107978 Third row of A121388.
%K A107978 easy,nonn
%O A107978 1,1
%A A107978 _Jonathan Vos Post_, Jun 12 2005
%E A107978 Edited by _N. J. A. Sloane_, May 20 2010