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A263239 Euler pseudoprimes to base 9: composite integers such that abs(9^((n - 1)/2)) == 1 mod n.

%I A263239 #17 Nov 17 2019 03:50:57
%S A263239 4,28,91,121,286,532,671,703,949,1036,1105,1541,1729,1891,2465,2665,
%T A263239 2701,2821,3281,3367,3751,4636,4961,5551,6364,6601,7381,8401,8911,
%U A263239 10585,11011,11476,12403,14383,15203,15457,15841,16471,16531,18721,19345,19684,23521,24046,24661,24727
%N A263239 Euler pseudoprimes to base 9: composite integers such that abs(9^((n - 1)/2)) == 1 mod n.
%C A263239 Even numbers are permitted since 9 is an integer square. - _Charles R Greathouse IV_, Oct 12 2015
%H A263239 Amiram Eldar, <a href="/A263239/b263239.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..116 from Daniel Lignon)
%t A263239 eulerPseudo9Q[n_]:=(Mod[9^((n-1)/2)+1,n]==0 ||Mod[9^((n-1)/2)-1,n]==0) && Not[PrimeQ[n]];
%t A263239 Select[Range[2,200000],eulerPseudo9Q]
%o A263239 (PARI) is(n) = abs(centerlift(Mod(3, n)^(n-1)))==1 && !isprime(n) && n>1 \\ _Charles R Greathouse IV_, Oct 12 2015
%Y A263239 Cf. A020138 (pseudoprimes to base 9).
%Y A263239 Cf. A006970 (base 2), A262051 (base 3), A262052 (base 5), A262053 (base 6), A262054 (base 7), A262055 (base 8).
%K A263239 nonn
%O A263239 1,1
%A A263239 _Daniel Lignon_, Oct 12 2015