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 A221048 #31 Feb 08 2021 23:22:03
%S A221048 21,39,55,57,93,111,129,155,183,201,203,205,219,237,253,291,301,305,
%T A221048 309,327,355,381,417,453,471,489,497,505,543,579,597,633,655,669,687,
%U A221048 689,723,737,755,791,813,831,849,889,905,921,939,955,979,993,1011,1027,1047
%N A221048 The odd semiprime numbers (A046315) which are orders of a non-Abelian group.
%C A221048 Numbers of the form pq where p,q are odd primes, p<q and q is congruent to 1 mod p.
%C A221048 The corresponding non-Abelian groups are the semidirect products of Z/qZ and Z/pZ. - _Bernard Schott_, May 16 2020
%H A221048 Jinyuan Wang, <a href="/A221048/b221048.txt">Table of n, a(n) for n = 1..1000</a>
%H A221048 S. K. Berberian, <a href="http://www.jstor.org/stable/2306479">Non-Abelian Groups of Order pq, American Mathematical Monthly</a>, Vol. 60, No. 1, Jan. 1953, 37-40.
%t A221048 Select[1 + 2*Range[500], (f = FactorInteger[#]; Last /@ f == {1, 1} && Mod @@ Reverse[First /@ f] == 1) &] (* _Giovanni Resta_, Apr 14 2013 *)
%o A221048 (PARI) lista(nn) = {forstep(n=1, nn, 2, my(f=factor(n)); if ((#f~ == 2) && (vecmax(f[,2]) == 1) && ((f[2,1] % f[1,1]) == 1), print1(n, ", ")););} \\ _Michel Marcus_, Sep 28 2017
%o A221048 (PARI) list(lim)=my(v=List()); if(lim<9, return([])); forprime(p=3,sqrtint(((lim\=1)-1)\2), forprimestep(q=2*p+1,lim,2*p, listput(v, p*q))); Set(v) \\ _Charles R Greathouse IV_, Feb 08 2021
%Y A221048 Intersection of A046315 and A060652.
%K A221048 nonn
%O A221048 1,1
%A A221048 _David Brown_, Apr 14 2013
%E A221048 More terms from _Jinyuan Wang_, May 16 2020