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 A307217 #38 Apr 11 2019 22:36:25
%S A307217 9,15,35,119,5543,74447,90859,110767,222179,389993,1526849,2927297,
%T A307217 3626699,4559939,24017531,137051711,160832099,229731743,627699239,
%U A307217 880021141,1001124539,1041287603,1104903617,1592658611,1717999139,8843679683,15575602979,15614760199,20374337479
%N A307217 Semiprimes p*q such that 2^(p+q) == 1 (mod p*q).
%C A307217 For k > 9, these are semiprimes k such that 2^(k+1) == 1 (mod k): semiprimes in A187787.
%C A307217 In this sequence, only 9 is a perfect square. - _Jinyuan Wang_, Mar 30 2019
%o A307217 (PARI) isok(k) = (bigomega(k)==2) && (Mod(2, k)^(k+1) == 1); \\ (for k > 9) _Michel Marcus_, Mar 29 2019
%o A307217 (Perl) use ntheory ":all"; forsemiprimes { print "$_\n" if powmod(2, vecsum(factor($_)), $_) == 1 } 4, 1e7; # _Daniel Suteu_, Mar 30 2019
%Y A307217 Cf. A001358, A046315, A127104, A187787, A208728.
%K A307217 nonn
%O A307217 1,1
%A A307217 _Thomas Ordowski_, Mar 29 2019
%E A307217 a(7)-a(18) from _Amiram Eldar_, Mar 29 2019
%E A307217 a(19)-a(29) from _Daniel Suteu_, Mar 29 2019