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 A293394 #35 Feb 09 2021 01:55:14 %S A293394 1,17,41,97,137,193,241,313,401,409,433,449,457,521,569,641,673,761, %T A293394 769,809,857,929,953,977,1009,1129,1297,1321,1361,1409,1489,1657,1697, %U A293394 1873,1993,2017,2081,2137,2153,2161 %N A293394 Numbers k such that (2*k-1)*(2^((k-1)/4)) == 1 (mod k). %C A293394 It appears that many elements of this sequence are prime. The first "pseudoprime" in this sequence is 74665. %H A293394 Charles R Greathouse IV, <a href="/A293394/b293394.txt">Table of n, a(n) for n = 1..10000</a> %H A293394 Jonas Kaiser, <a href="https://arxiv.org/abs/1608.00862">On the relationship between the Collatz conjecture and Mersenne prime numbers</a>, arXiv:1608.00862 [math.GM], 2016. %t A293394 Select[Range[1, 3001, 4], #==1 || Mod[-PowerMod[#-2, (#-1)/4, #], #]==1&] (* _Jean-François Alcover_, Nov 18 2018 *) %o A293394 (PARI) is(n)=n%4==1 && (2*n-1)*Mod(2,n)^(n>>2)==1 \\ _Charles R Greathouse IV_, Nov 09 2017 %Y A293394 Cf. A001133, A244626, A294717. %K A293394 nonn %O A293394 1,2 %A A293394 _Jonas Kaiser_, Nov 09 2017