OFFSET
1,1
COMMENTS
For each prime p, Fibonacci(p) = 5^((p-1)/2) mod p, so p divides Fibonacci(p) + 1 for each prime p=10k+-3. Hence it is interesting to seek also nonprimes with the same property, a motivation for this sequence. - Robert FERREOL, Jul 14 2015
Are all terms squarefree? A counterexample can't be divisible by the square of a prime < 1000. - Robert Israel, Jul 17 2015
Any term that is not squarefree must be divisible by the square of a Fibonacci-Wieferich prime (see the StackExchange link). There are believed to be infinitely many such primes, but none are known, and none are less than 2*10^14. - Robert Israel, Jul 22 2015
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..579 (terms < 4*10^9)
R. Israel and N. Elkies, Fibonacci == -1 mod p^2, Mathematics StackExchange, July 2015.
R. J. McIntosh and E. L. Roettger, A search for Fibonacci-Wieferich and Wolstenholme primes, Math. Comp. 76 (2007), 2087-2094.
MAPLE
with(combinat):test:=n->(fibonacci(n)+1) mod n= 0:
select(test and not isprime , [seq(2*k+1, k=1..10000)]);
# Robert FERREOL, Jul 14 2015
MATHEMATICA
Select[ Range[3, 300000, 2], !PrimeQ[ # ] && Mod[Fibonacci[ # ] + 1, # ] == 0 &]
PROG
(PARI) main(size)=my(v=vector(size), i, t=1); for(i=1, size, while(1, if(t%2==1&&omega(t)>1&&(fibonacci(t)+1)%t==0, v[i]=t; break, t++)); t++); v; \\ Anders Hellström, Jul 17 2015
(PARI) is(n)=((Mod([1, 1; 1, 0], n))^n)[1, 2]==-1 && n%2 && !isprime(n) \\ Charles R Greathouse IV, Jul 20 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Eric Rowland, May 01 2004
EXTENSIONS
a(6)-a(14) from Robert G. Wilson v, May 01 2004
More terms from Ryan Propper, Aug 03 2005
STATUS
approved