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 A367231 #18 Apr 22 2025 05:21:35 %S A367231 15841,52633,5049001,68154001,104852881,238244041,382536001, %T A367231 3215031751,3863326897,7211236033,8214723001,15462960481,22008493921, %U A367231 23000028481,29392867201,31708772257,41217865921,53125756201,60518537641,74190097801,77874636001,83828294551,103387371361 %N A367231 Carmichael numbers k such that the multiplicative order of 2 modulo k is odd. %C A367231 These Carmichael numbers seem to be relatively rare: among the 4279356 Carmichael numbers below 2^64 only 3097 are terms of this sequence. %H A367231 Amiram Eldar, <a href="/A367231/b367231.txt">Table of n, a(n) for n = 1..10000</a> (calculated using data from Claude Goutier) %H A367231 Claude Goutier, <a href="http://www-labs.iro.umontreal.ca/~goutier/OEIS/A055553/">Compressed text file carm10e22.gz containing all the Carmichael numbers up to 10^22</a>. %H A367231 <a href="/index/Ca#Carmichael">Index entries for sequences related to Carmichael numbers</a>. %t A367231 Select[2*Range[3*10^6] + 1, Mod[#, CarmichaelLambda[#]] == 1 && CompositeQ[#] && OddQ[MultiplicativeOrder[2, #]] &] %o A367231 (PARI) is(n) = n > 1 && n % 2 && !isprime(n) && n % lcm(znstar(n)[2]) == 1 && znorder(Mod(2, n)) % 2; %Y A367231 Intersection of A002997 and A036259. %Y A367231 Subsequence of A367230. %Y A367231 Cf. A002326, A163956. %K A367231 nonn %O A367231 1,1 %A A367231 _Amiram Eldar_, Nov 11 2023