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A081264
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Odd Fibonacci pseudoprimes: odd composite numbers k such that either (1) k divides Fibonacci(k-1) if k == +-1 (mod 5) or (2) k divides Fibonacci(k+1) if k == +-2 (mod 5).
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21
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323, 377, 1891, 3827, 4181, 5777, 6601, 6721, 8149, 10877, 11663, 13201, 13981, 15251, 17119, 17711, 18407, 19043, 23407, 25877, 27323, 30889, 34561, 34943, 35207, 39203, 40501, 50183, 51841, 51983, 52701, 53663, 60377, 64079, 64681
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OFFSET
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1,1
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COMMENTS
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Lehmer shows that there are an infinite number of Fibonacci pseudoprimes (FPPs). In particular, the number Fibonacci(2p) is an FPP for all primes p > 5. Anderson lists over 5000 FPPs, while Jacobsen lists over 170000. The sequences A069106 and A069107 give k such that k divides Fibonacci(k-1) and k divides Fibonacci(k+1), respectively. See A141137 for even FPPs.
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REFERENCES
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R. Crandall and C. Pomerance, Primes Numbers: A Computational Perspective, Springer, 2002, p. 131.
P. Ribenboim, The New Book of Prime Number Records, Springer, 1995, p. 127.
A. Witno, Theory of Numbers, BookSurge, North Charleston, SC; see p. 83.
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LINKS
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MAPLE
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filter:= proc(n) local M, r;
uses LinearAlgebra:-Modular;
if isprime(n) then return false fi;
M:= Mod(n, [[1, 1], [1, 0]], float[8]);
if n^2 mod 5 = 1 then r:= n-1 else r:= n+1 fi;
M:= MatrixPower(n, M, r);
M[1, 2] = 0
end proc:select(filter, [2*i+1 $ i=1..10^5]); # Robert Israel, Aug 05 2015
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MATHEMATICA
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lst={}; f0=0; f1=1; Do[f2=f1+f0; If[n>1&&!PrimeQ[n], If[MemberQ[{1, 4}, Mod[n, 5]], If[Mod[f0, n]==0, AppendTo[lst, n]]]; If[MemberQ[{2, 3}, Mod[n, 5]], If[Mod[f2, n]==0, AppendTo[lst, n]]]]; f0=f1; f1=f2, {n, 100000}]; lst
ocnQ[n_]:=CompositeQ[n]&&Which[Mod[n, 5]==1, Divisible[Fibonacci[ n-1], n], Mod[n, 5] == 4, Divisible[ Fibonacci[n-1], n], Mod[n, 5]==2, Divisible[ Fibonacci[n+1], n], Mod[n, 5]==3, Divisible[Fibonacci[n+1], n], True, False]; Select[Range[1, 65001, 2], ocnQ] (* Harvey P. Dale, Aug 23 2017 *)
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PROG
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(Perl) use ntheory ":all"; foroddcomposites { $e = (0, -1, 1, 1, -1)[$_%5]; say unless $e==0 || (lucas_sequence($_, 1, -1, $_+$e))[0] } 1e10; # Dana Jacobsen, Aug 05 2015
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CROSSREFS
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KEYWORD
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nice,nonn
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AUTHOR
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STATUS
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approved
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