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 A178723 #15 Sep 10 2019 04:52:25 %S A178723 341,561,645,1105,1729,1905,2465,2701,2821,3277,4033,4369,4681,5461, %T A178723 6601,7957,8321,8481,10261,10585,11305,12801,13741,13981,15709,15841, %U A178723 16705,18705,18721,23001,23377,25761,29341,30121,30889,31417,31609,31621,33153,34945,35333,39865,41041,41665,46657,49141,49981,52633 %N A178723 Pseudoprimes to base 2 of the form 4k+1. %H A178723 Amiram Eldar, <a href="/A178723/b178723.txt">Table of n, a(n) for n = 1..10000</a> %t A178723 Select[4 * Range[10^4] + 1, CompositeQ[#] && PowerMod[2, # - 1, #] == 1 &] (* _Amiram Eldar_, Sep 10 2019 *) %o A178723 (PARI) forstep(n=5, 10^7, 4, if(isprime(n), next()); if(Mod(2, n)^(n-1)==1, print1(n, ", "))) %Y A178723 Cf. A001567, A177884. %K A178723 nonn %O A178723 1,1 %A A178723 _Joerg Arndt_, Dec 30 2010