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Composite n such that F(n^2+1)==1 (mod n) where F(k) denotes the k-th Fibonacci number.
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%I #12 Aug 03 2020 15:29:19

%S 4,9,10,12,20,24,27,28,36,48,50,60,63,72,76,84,96,99,100,108,110,120,

%T 140,144,161,168,180,192,196,204,216,220,228,231,240,250,252,276,288,

%U 300,323,324,336,341,351,360,364,369,377,384,408,420,432,451,456,480

%N Composite n such that F(n^2+1)==1 (mod n) where F(k) denotes the k-th Fibonacci number.

%t Select[Range[500],CompositeQ[#]&&Mod[Fibonacci[#^2+1],#]==1&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Aug 03 2020 *)

%o (PARI) isok(n) = !isprime(n) && ((fibonacci(n^2+1) % n) == 1); \\ _Michel Marcus_, Dec 06 2013

%Y Cf. A086367.

%Y Subsequence of A086391.

%K nonn

%O 1,1

%A _Benoit Cloitre_, Sep 06 2003