# Fibonacci prime

A Fibonacci prime is a Fibonacci number that is prime, a type of integer sequence prime.

No. of known terms 51 Infinite 2, 3, 5, 13, 89, 233 F3340367 A001605Indices of prime Fibonacci numbers

The first Fibonacci primes are (sequence A005478 in the OEIS):

2, 3, 5, 13, 89, 233, 1597, 28657, 514229, 433494437, 2971215073, ....

## Known Fibonacci primes Unsolved problem in mathematics:Are there an infinite number of Fibonacci primes?(more unsolved problems in mathematics)

It is not known whether there are infinitely many Fibonacci primes. With the indexing starting with F1 = F2 = 1, the first 34 are Fn for the n values (sequence A001605 in the OEIS):

n = 3, 4, 5, 7, 11, 13, 17, 23, 29, 43, 47, 83, 131, 137, 359, 431, 433, 449, 509, 569, 571, 2971, 4723, 5387, 9311, 9677, 14431, 25561, 30757, 35999, 37511, 50833, 81839, 104911.

In addition to these proven Fibonacci primes, there have been found probable primes for

n = 130021, 148091, 201107, 397379, 433781, 590041, 593689, 604711, 931517, 1049897, 1285607, 1636007, 1803059, 1968721, 2904353, 3244369, 3340367.

Except for the case n = 4, all Fibonacci primes have a prime index, because if a divides b, then $F_{a}$  also divides $F_{b}$ , but not every prime is the index of a Fibonacci prime.

Fp is prime for 8 of the first 10 primes p; the exceptions are F2 = 1 and F19 = 4181 = 37 × 113. However, Fibonacci primes appear to become rarer as the index increases. Fp is prime for only 26 of the 1,229 primes p below 10,000. The number of prime factors in the Fibonacci numbers with prime index are:

0, 1, 1, 1, 1, 1, 1, 2, 1, 1, 2, 3, 2, 1, 1, 2, 2, 2, 3, 2, 2, 2, 1, 2, 4, 2, 3, 2, 2, 2, 2, 1, 1, 3, 4, 2, 4, 4, 2, 2, 3, 3, 2, 2, 4, 2, 4, 4, 2, 5, 3, 4, 3, 2, 3, 3, 4, 2, 2, 3, 4, 2, 4, 4, 4, 3, 2, 3, 5, 4, 2, 1, ... (sequence A080345 in the OEIS)

As of March 2017, the largest known certain Fibonacci prime is F104911, with 21925 digits. It was proved prime by Mathew Steine and Bouk de Water in 2015. The largest known probable Fibonacci prime is F3340367. It was found by Henri Lifchitz in 2018. It was proved by Nick MacKinnon that the only Fibonacci numbers that are also members of the set of twin primes are 3, 5, and 13.

## Divisibility of Fibonacci numbers

A prime $p$  divides $F_{p-1}$  if and only if p is congruent to ±1 modulo 5, and p divides $F_{p+1}$  if and only if it is congruent to ±2 modulo 5. (For p = 5, F5 = 5 so 5 divides F5)

Fibonacci numbers that have a prime index p do not share any common divisors greater than 1 with the preceding Fibonacci numbers, due to the identity:

$\gcd(F_{n},F_{m})=F_{\gcd(n,m)},$

which implies the infinitude of primes.

For n ≥ 3, Fn divides Fm iff n divides m.

If we suppose that m is a prime number p, and n is less than p, then it is clear that Fp, cannot share any common divisors with the preceding Fibonacci numbers.

$\gcd(F_{p},F_{n})=F_{\gcd(p,n)}=F_{1}=1.$

This means that Fp will always have characteristic factors or be a prime characteristic factor itself. The number of distinct prime factors of each Fibonacci number can be put into simple terms.

• Fnk is a multiple of Fk for all values of n and k from 1 up. It's safe to say that Fnk will have "at least" the same number of distinct prime factors as Fk. All Fp will have no factors of Fk, but "at least" one new characteristic prime from Carmichael's theorem.
• Carmichael's Theorem applies to all Fibonacci numbers except 4 special cases: $F_{1}=F_{2}=1,F_{6}=8$  and $F_{12}=144.$  If we look at the prime factors of a Fibonacci number, there will be at least one of them that has never before appeared as a factor in any earlier Fibonacci number. Let πn be the number of distinct prime factors of Fn. (sequence A022307 in the OEIS)
If k | n then $\pi _{n}\geqslant \pi _{k}+1$  except for $\pi _{6}=\pi _{3}=1.$
If k = 1, and n is an odd prime, then 1 | p and $\pi _{p}\geqslant \pi _{1}+1=1.$
 n Fn πn 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765 10946 17711 28657 46368 75025 0 0 0 1 1 1 1 1 2 2 2 1 2 1 2 3 3 1 3 2 4 3 2 1 4 2

The first step in finding the characteristic quotient of any Fn is to divide out the prime factors of all earlier Fibonacci numbers Fk for which k | n.

The exact quotients left over are prime factors that have not yet appeared.

If p and q are both primes, then all factors of Fpq are characteristic, except for those of Fp and Fq.

{\begin{aligned}\gcd(F_{pq},F_{q})&=F_{\gcd(pq,q)}=F_{q}\\\gcd(F_{pq},F_{p})&=F_{\gcd(pq,p)}=F_{p}\end{aligned}}

Therefore:

$\pi _{pq}\geqslant {\begin{cases}\pi _{p}+\pi _{q}+1&p\neq q\\\pi _{p}+1&p=q\end{cases}}$

The number of distinct prime factors of the Fibonacci numbers with a prime index is directly relevant to the counting function. (sequence A080345 in the OEIS)

 p πp 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 0 1 1 1 1 1 1 2 1 1 2 3 2 1 1 2 2 2 3 2 2 2 1 2 4

## Rank of Apparition

For a prime p, the smallest index u > 0 such that Fu is divisible by p is called the rank of apparition (sometimes called Fibonacci entry point) of p and denoted a(p). The rank of apparition a(p) is defined for every prime p. The rank of apparition divides the Pisano period π(p) and allows to determine all Fibonacci numbers divisible by p.

For the divisibility of Fibonacci numbers by powers of a prime, $p\geqslant 3,n\geqslant 2$  and $k\geqslant 0$

$p^{n}\mid F_{a(p)kp^{n-1}}.$

In particular

$p^{2}\mid F_{a(p)p}.$

## Wall-Sun-Sun primes

A prime p ≠ 2, 5 is called a Fibonacci–Wieferich prime or a Wall-Sun-Sun prime if $p^{2}\mid F_{q},$  where

$q=p-\left({\frac {p}{5}}\right)$

in which $\left({\tfrac {p}{5}}\right)$  is the Legendre symbol defined as:

$\left({\frac {p}{5}}\right)={\begin{cases}1&p\equiv \pm 1{\bmod {5}}\\-1&p\equiv \pm 2{\bmod {5}}\end{cases}}$

It is known that for p ≠ 2, 5, a(p) is a divisor of:

$p-\left({\frac {p}{5}}\right)={\begin{cases}p-1&p\equiv \pm 1{\bmod {5}}\\p+1&p\equiv \pm 2{\bmod {5}}\end{cases}}$

For every prime p that is not a Wall-Sun-Sun prime, $a(p^{2})=pa(p)$  as illustrated in the table below:

 p a(p) a(p2) 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 3 4 5 8 10 7 9 18 24 14 30 19 20 44 16 27 58 15 6 12 25 56 110 91 153 342 552 406 930 703 820 1892 752 1431 3422 915

The existence of Wall-Sun-Sun primes is conjectural.

## Fibonacci primitive part

The primitive part of the Fibonacci numbers are

1, 1, 2, 3, 5, 4, 13, 7, 17, 11, 89, 6, 233, 29, 61, 47, 1597, 19, 4181, 41, 421, 199, 28657, 46, 15005, 521, 5777, 281, 514229, 31, 1346269, 2207, 19801, 3571, 141961, 321, 24157817, 9349, 135721, 2161, 165580141, 211, 433494437, 13201, 109441, ... (sequence A061446 in the OEIS)

The product of the primitive prime factors of the Fibonacci numbers are

1, 1, 2, 3, 5, 1, 13, 7, 17, 11, 89, 1, 233, 29, 61, 47, 1597, 19, 4181, 41, 421, 199, 28657, 23, 3001, 521, 5777, 281, 514229, 31, 1346269, 2207, 19801, 3571, 141961, 107, 24157817, 9349, 135721, 2161, 165580141, 211, 433494437, 13201, 109441, 64079, 2971215073, 1103, 598364773, 15251, ... (sequence A178763 in the OEIS)

The first case of more than one primitive prime factor is 4181 = 37 × 113 for $F_{19}$ .

The primitive part has a non-primitive prime factor in some cases. The ratio between the two above sequences is

1, 1, 1, 1, 1, 4, 1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 5, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 7, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 13, 1, 1, .... (sequence A178764 in the OEIS)

The natural numbers n for which $F_{n}$  has exactly one primitive prime factor are

3, 4, 5, 7, 8, 9, 10, 11, 13, 14, 15, 16, 17, 18, 20, 21, 22, 23, 24, 25, 26, 28, 29, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 43, 45, 47, 48, 51, 52, 54, 56, 60, 62, 63, 65, 66, 72, 74, 75, 76, 82, 83, 93, 94, 98, 105, 106, 108, 111, 112, 119, 121, 122, 123, 124, 125, 131, 132, 135, 136, 137, 140, 142, 144, 145, ... (sequence A152012 in the OEIS)

If and only if a prime p is in this sequence, then $F_{p}$  is a Fibonacci prime, and if and only if 2p is in this sequence, then $L_{p}$  is a Lucas prime (where $L_{n}$  is the Lucas sequence), and if and only if 2n is in this sequence, then $L_{2^{n-1}}$  is a Lucas prime.

Number of primitive prime factors of $F_{n}$  are

0, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1, 1, 2, 1, 2, 1, 1, 2, 2, 1, 1, 2, 1, 2, 1, 2, 2, 2, 1, 2, 1, 1, 2, 1, 1, 3, 2, 3, 2, 2, 1, 2, 1, 1, 1, 2, 2, 2, 2, 3, 1, 1, 2, 2, 2, 2, 3, 2, 2, 2, 2, 1, 1, 3, 2, 4, 1, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 2, 2, 1, 1, 2, 2, 2, 2, 2, 3, 1, 2, 1, 1, 1, 1, 1, 2, 2, 2, ... (sequence A086597 in the OEIS)

The least primitive prime factor of $F_{n}$  are

1, 1, 2, 3, 5, 1, 13, 7, 17, 11, 89, 1, 233, 29, 61, 47, 1597, 19, 37, 41, 421, 199, 28657, 23, 3001, 521, 53, 281, 514229, 31, 557, 2207, 19801, 3571, 141961, 107, 73, 9349, 135721, 2161, 2789, 211, 433494437, 43, 109441, 139, 2971215073, 1103, 97, 101, ... (sequence A001578 in the OEIS)