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 A175942 #38 Jun 25 2021 16:15:41 %S A175942 5,11,23,47,59,83,107,167,179,227,263,347,359,383,467,479,503,563,587, %T A175942 683,719,839,863,887,983,1019,1187,1283,1307,1319,1367,1439,1487,1523, %U A175942 1619,1823,1907,2027,2039,2063,2099,2207,2447,2459,2543,2579,2819,2879 %N A175942 Odd numbers k such that 4^k == 4 (mod 3*k) and 2^(k-1) == 4 (mod 3*(k-1)). %C A175942 Equivalently, integers k == 5 (mod 6) such that 4^k == 4 (mod k) and 2^(k-1) == 4 (mod k-1). %C A175942 Equivalently, integers k == 5 (mod 6) such that both k and (k-1)/2 are primes or (odd or even) Fermat 4-pseudoprimes (A122781). %C A175942 Contains terms k of A175625 such that k == 5 (mod 6). %C A175942 Contains terms k of A303448 such that k == 5 (mod 6). %C A175942 Many composite terms of this sequence are of the form A007583(m) = (2^(2m+1) + 1)/3 (for m in A303009). It is unknown if there exist composite terms not of this form. %C A175942 Numbers k such that 2^(k-1) == 3k+1 (mod 3(k-1)k). This sequence contains all safe primes except 7. The term a(20) = 683 = 2*341+1 is the smallest prime that is not safe. - _Thomas Ordowski_, Jun 07 2021 %H A175942 Harvey P. Dale, <a href="/A175942/b175942.txt">Table of n, a(n) for n = 1..1000</a> %t A175942 Select[Range[1,3001,2],PowerMod[4,#,3#]==4&&PowerMod[2,#-1,3(#-1)]==4&] (* _Harvey P. Dale_, Aug 04 2018 *) %Y A175942 Cf. A005385. %K A175942 nonn %O A175942 1,1 %A A175942 _Alzhekeyev Ascar M_, Oct 27 2010 %E A175942 Edited by _Max Alekseyev_, Apr 24 2018