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 A216667 #22 Feb 16 2025 08:33:18 %S A216667 1387,2047,3277,7957,13747,23377,31417,60787,65077,88357,164737, %T A216667 188057,233017,275887,390937,486737,489997,514447,580337,604117, %U A216667 672487,680627,769567,769757,916327,1092547,1132657,1145257,1252697,1293337,1433407,1493857,1530787 %N A216667 Semiprime 2-pseudoprimes of the form 10k + 7. %C A216667 A very interesting observation due to _Peter Bala_: about half of the terms from the sequence have the form p*(4*p - 3), where p is prime. For this form of Fermat pseudoprimes see the sequences A213812 and A215343. %H A216667 Charles R Greathouse IV, <a href="/A216667/b216667.txt">Table of n, a(n) for n = 1..10000</a> %H A216667 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PouletNumber.html">Poulet Number</a> %o A216667 (PARI) list(lim)=my(v=List(),t); forprime(p=3,sqrtint(lim\=1), forprime(q=p+2,lim\p, t=p*q; if(t%10==7 && Mod(2,t)^t==2, listput(v,t)))); Set(v) \\ _Charles R Greathouse IV_, Jun 30 2017 %Y A216667 Subsequence of A214305. %Y A216667 Cf. A001567. %K A216667 nonn %O A216667 1,1 %A A216667 _Marius Coman_, Sep 13 2012