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%I #41 Apr 03 2023 10:36:09
%S 15,85,91,341,435,451,561,645,703,1105,1247,1271,1387,1581,1695,1729,
%T 1891,1905,2047,2071,2465,2701,2821,3133,3277,3367,3683,4033,4369,
%U 4371,4681,4795,4859,5461,5551,6601,6643,7957,8321,8481,8695,8911,9061,9131
%N Fermat pseudoprimes to base 4.
%C 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 Primes q and 2q-1 are a Cunningham chain of the second kind. - _Walter Nissen_, Sep 07 2009
%C Composite numbers n such that 4^(n-1) == 1 (mod n). - _Michel Lagneau_, Feb 18 2012
%H 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 Chris Caldwell, <a href="https://t5k.org/glossary/xpage/CunninghamChain.html"> Cunningham chain</a>.
%H Chris Caldwell et al., <a href="https://t5k.org/top20/page.php?id=20"> Top Twenty Cunningham Chains (2nd kind)</a>.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a>.
%H <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>
%t Select[Range[9200], ! PrimeQ[ # ] && PowerMod[4, # - 1, # ] == 1 &] (* _Farideh Firoozbakht_, Sep 12 2006 *)
%o (PARI) isok(n) = (Mod(4, n)^(n-1)==1) && !isprime(n) && (n>1); \\ _Michel Marcus_, Apr 27 2018
%Y Subsequence of A122781.
%Y Contains A001567 (Fermat pseudoprimes to base 2) as a subsequence.
%Y Cf. A005382, A129521.
%K nonn
%O 1,1
%A _David W. Wilson_
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