# Fermat number

A Fermat number, named after Pierre de Fermat who first studied them, is a positive integer of the form

$F_{n}=2^{2^{n}}+1$ where n is a nonnegative integer. The first eight Fermat numbers are

F0 = 21 + 1 = 3
F1 = 22 + 1 = 5
F2 = 24 + 1 = 17
F3 = 28 + 1 = 257
F4 = 216 + 1 = 65537
F5 = 232 + 1 = 4294967297 = 641 * 6700417
F6 = 264 + 1 = 18446744073709551617 = 274177 * 67280421310721
F7 = 2128 + 1 = 340282366920938463463374607431768211457 = 59649589127497217 · 5704689200685129054721

See also . If 2n + 1 is prime, it can be shown that n must be a power of 2. In other words, every prime of the form 2n + 1 is a Fermat number, and such primes are called Fermat primes. The only known Fermat primes are F0,...,F4.

## Basic Properties

The Fermat numbers satisfy the following recurrence relations

$F_{n}=(F_{n-1}-1)^{2}+1$ $F_{n}=F_{n-1}+2^{2^{n-1}}F_{0}\cdots F_{n-2}$ $F_{n}=F_{n-1}^{2}-2(F_{n-2}-1)^{2}$ $F_{n}=F_{0}\cdots F_{n-1}+2$ for n ≥ 2. Each of these relations can be proved by mathematical induction. From the last equation, we can deduce Goldbach's theorem: no two Fermat numbers share a common factor. To see this, suppose that 0 ≤ i < j and Fi and Fj have a common factor a > 1. Then a divides both

$F_{0}\cdots F_{j-1}$ and Fj; hence a divides their difference 2. Since a > 1, this forces a = 2. This is a contradiction, because each Fermat number is clearly odd. As a corollary, we obtain another proof of the infinitude of the prime numbers: for each Fn, choose a prime factor pn; then the sequence {pn} is an infinite sequence of distinct primes.

Here are some other basic properties of the Fermat numbers:

• If n ≥ 2, then Fn ≡ 17 or 41 (mod 72). (See modular arithmetic)
• If n ≥ 2, then Fn ≡ 17, 37, 57, or 97 (mod 100).
• The number of digits D(n,b) of Fn expressed in the base b is
$D(n,b)=\lfloor \log _{b}\left(2^{2^{n}}+1\right)+1\rfloor \approx \lfloor 2^{n}\,\log _{b}2+1\rfloor$ (See floor function)
• No Fermat number can be expressed as the sum of two primes.
• No Fermat prime can be expressed as the difference of two pth powers, where p is an odd prime.

## Primality of Fermat numbers

Fermat numbers and Fermat primes were first studied by Pierre de Fermat, who conjectured that all Fermat numbers are prime. Indeed, the first five Fermat numbers F0,...,F4 are easily shown to be prime. However, this conjecture was refuted by Leonhard Euler in 1732 when he showed that

$F_{5}=2^{2^{5}}+1=2^{32}+1=4294967297=641\cdot 6700417\;$ It is interesting to note how Euler found this factorization. Euler had proved that every factor of Fn must have the form k2n+1 + 1. For n = 5, this means that the only possible factors are of the form 64k + 1. It did not take Euler very long to find the factor 641 = 10×64 + 1.

It is widely believed that Fermat was aware of Euler's result, so it seems curious why he failed to follow through on the straightforward calculation to find the factor. One common explanation is that Fermat made a computational mistake and was so convinced of the correctness of his claim that he failed to double-check his work.

There are no other known Fermat primes Fn with n > 4. In fact, each of the following is an open problem:

• Is Fn composite for all n > 4?
• Are there infinitely many Fermat primes?
• Are there infinitely many composite Fermat numbers?

The following heuristic argument suggests there are only finitely many Fermat primes: according to the prime number theorem, the "probability" that a number n is prime is at most A/ln(n), where A is a fixed constant. Therefore, the total expected number of Fermat primes is at most

$A\sum _{n=0}^{\infty }{\frac {1}{\ln F_{n}}}={\frac {A}{\ln 2}}\sum _{n=0}^{\infty }{\frac {1}{\log _{2}(2^{2^{n}}+1)}}<{\frac {A}{\ln 2}}\sum _{n=0}^{\infty }2^{-n}={\frac {2A}{\ln 2}}$ It should be stressed that this argument is in no way a rigorous proof. For one thing, the argument assumes that Fermat numbers behave "randomly", yet we have already seen that the factors of Fermat numbers have special properties. Although it is widely believed that there are only finitely many Fermat primes, it should be noted that there are some experts who disagree. 

As of this writing (2004), it is known that Fn is composite for 5 ≤ n ≤ 32, although complete factorisations of Fn are known only for 0 ≤ n ≤ 11. The largest known composite Fermat number is F2478785, and its prime factor 3×22478785 + 1 was discovered by John Cosgrave and his Proth-Gallot Group on October 10, 2003.

There are a number of conditions that are equivalent to the primality of Fn.

• Proth's theorem -- (1878) Let N = k(2m + 1) with odd k < 2m. If there is an integer a such that
$a^{(N-1)/2}\equiv -1\mod N$ then N is prime. Conversely, if the above congruence does not hold, and in addition
$\left({\frac {a}{N}}\right)=-1$ (See Jacobi symbol)
then N is composite. If N = Fn > 3, then the above Jacobi symbol is always equal to −1 for a = 3, and this special case of Proth's theorem is known as Pepin's test. Although Pepin's test and Proth's theorem have been implemented on computers to prove the compositeness of many Fermat numbers, neither test gives a specific nontrivial factor. In fact, no specific prime factors are known for n = 14, 20, 22, and 24.
• The Fermat number Fn is prime if and only if φ(n) is a power of 2, where φ(n) is Euler's totient function.
• Let n ≥ 3 be a positive odd integer. Then n is a Fermat prime if and only if for every a coprime to n, a is a primitive root mod n if and only if a is a quadratic nonresidue mod n.
• The Fermat number Fn > 3 is prime if and only if it can be written uniquely as a sum of two nonzero squares, namely
$F_{n}=\left(2^{2^{n-1}}\right)^{2}+1^{2}$ For example, F5 = 622642 + 204492 and F6 = 40468032562 + 14387937592.

## Factorisation of Fermat numbers

...Lucas's theorem...Sierpinski number...

## Fermat's little theorem and pseudoprimes

...Using Fermat numbers to generate infinitely many pseudoprimes...

## Relationship to Constructible Polygons

An n-sided regular polygon can be constructed with ruler and compass if n is a power of 2 or the product of a power of 2 and distinct Fermat primes. In other words, if n is of the form n = 2kp1p2...ps, where k is a nonnegative integer and the pi are distinct Fermat primes. See constructible polygon.

A positive integer n is of the above form if and only if φ(n) is a power of 2, where φ(n) is Euler's totient function.

## Applications of Fermat Numbers

...Fermat number transform...random number generation...

## Other interesting facts

...F_n cannot be a perfect power, perfect, or part of amicable pair, etc...

## Generalised Fermat numbers

...brief definition of L(p,m) and G(p,m)...