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 A020136 #42 Feb 16 2025 08:32:33 %S A020136 15,85,91,341,435,451,561,645,703,1105,1247,1271,1387,1581,1695,1729, %T A020136 1891,1905,2047,2071,2465,2701,2821,3133,3277,3367,3683,4033,4369, %U A020136 4371,4681,4795,4859,5461,5551,6601,6643,7957,8321,8481,8695,8911,9061,9131 %N A020136 Fermat pseudoprimes to base 4. %C A020136 If q and 2q-1 are odd primes, then n=q*(2q-1) is in the sequence. So for n>1, A005382(n)*(2*A005382(n)-1) form a subsequence (cf. A129521). - _Farideh Firoozbakht_, Sep 12 2006 %C A020136 Primes q and 2q-1 are a Cunningham chain of the second kind. - _Walter Nissen_, Sep 07 2009 %C A020136 Composite numbers n such that 4^(n-1) == 1 (mod n). - _Michel Lagneau_, Feb 18 2012 %H A020136 Amiram Eldar, <a href="/A020136/b020136.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe) %H A020136 Chris Caldwell, <a href="https://t5k.org/glossary/xpage/CunninghamChain.html"> Cunningham chain</a>. %H A020136 Chris Caldwell et al., <a href="https://t5k.org/top20/page.php?id=20"> Top Twenty Cunningham Chains (2nd kind)</a>. %H A020136 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a>. %H A020136 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a> %t A020136 Select[Range[9200], ! PrimeQ[ # ] && PowerMod[4, # - 1, # ] == 1 &] (* _Farideh Firoozbakht_, Sep 12 2006 *) %o A020136 (PARI) isok(n) = (Mod(4, n)^(n-1)==1) && !isprime(n) && (n>1); \\ _Michel Marcus_, Apr 27 2018 %Y A020136 Subsequence of A122781. %Y A020136 Contains A001567 (Fermat pseudoprimes to base 2) as a subsequence. %Y A020136 Cf. A005382, A129521. %K A020136 nonn %O A020136 1,1 %A A020136 _David W. Wilson_