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%I #20 Jun 29 2019 08:58:52
%S 561,1729,2821,5461,6601,8911,12801,13981,15841,29341,41041,46657,
%T 52633,63973,68101,75361,101101,113201,115921,126217,137149,162401,
%U 172081,188461,252601,294409,314821,334153,340561,399001,401401,410041,488881
%N Pseudoprimes to bases 2 and 5.
%H Amiram Eldar, <a href="/A083732/b083732.txt">Table of n, a(n) for n = 1..16699</a> (terms 1..169 from R. J. Mathar)
%H F. Richman, <a href="http://math.fau.edu/Richman/carm.htm">Primality testing with Fermat's little theorem</a>
%F a(n) = n-th positive integer k(>1) such that 2^(k-1) = 1 (mod k) and 5^(k-1) = 1 (mod k).
%e a(1)=561 since 561 is the first positive integer k(>1) which satisfies 2^(k-1) = 1 (mod k) and 5^(k-1) = 1 (mod k).
%t Select[Range[1, 10^5, 2], CompositeQ[#] && PowerMod[2, #-1,#] == PowerMod[5, #-1,#] == 1 &] (* _Amiram Eldar_, Jun 29 2019 *)
%o (PARI) lista(nn) = forcomposite(n=1, nn, if ((Mod(2, n)^(n-1)==1) && (Mod(5, n)^(n-1)==1), print1(n, ", "));); \\ _Michel Marcus_, Sep 08 2016
%Y Intersection of A001567 and A005936. - _R. J. Mathar_, Apr 05 2011
%K easy,nonn
%O 1,1
%A Serhat Sevki Dincer (sevki(AT)ug.bilkent.edu.tr), May 05 2003
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