# Fermat number

Fermat number

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

:$F_\left\{n\right\} = 2^\left\{2^\left\{ overset\left\{n\right\} \left\{\right\} + 1$

where "n" is a nonnegative integer. The first nine Fermat numbers are OEIS|id=A000215:

As of|2008, only the first 12 Fermat numbers have been completely factored.Wilfrid Keller, [http://www.prothsearch.net/fermat.html "Prime Factors of Fermat Numbers"] . Retrieved 2008-09-07.]

If 2"n" + 1 is prime, and "n" > 0, it can be shown that "n" must be a power of two. (If "n" = "ab" where 1 ≤ "a", "b" ≤ "n" and "b" is odd, then 2"n" + 1 ≡ (2"a")"b" + 1 ≡ (−1)"b" + 1 ≡ 0 (mod 2"a" + 1). See below for complete proof.) In other words, every prime of the form 2"n" + 1 is a Fermat number, and such primes are called Fermat primes. The only known Fermat primes are "F"0, "F"1, "F"2, "F"3, and "F"4.

Basic properties

The Fermat numbers satisfy the following recurrence relations

:$F_\left\{n\right\} = \left(F_\left\{n-1\right\}-1\right)^\left\{2\right\}+1,$:$F_\left\{n\right\} = F_\left\{n-1\right\} + 2^\left\{2^\left\{n-1F_\left\{0\right\} cdots F_\left\{n-2\right\}$:$F_\left\{n\right\} = F_\left\{n-1\right\}^2 - 2\left(F_\left\{n-2\right\}-1\right)^2$:$F_\left\{n\right\} = F_\left\{0\right\} cdots F_\left\{n-1\right\} + 2$

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 "F""i" and "F""j" have a common factor "a" > 1. Then "a" divides both

:$F_\left\{0\right\} cdots F_\left\{j-1\right\}$

and "F""j"; 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 "F""n", choose a prime factor "p""n"; then the sequence {"p""n"} is an infinite sequence of distinct primes.

Further properties:

*The number of digits "D"("n","b") of "F""n" expressed in the base "b" is

:$D\left(n,b\right) = lfloor log_\left\{b\right\}left\left(2^\left\{2^\left\{overset\left\{n\right\}\left\{\right\}+1 ight\right)+1 floor approx lfloor 2^\left\{n\right\},log_\left\{b\right\}2+1 floor$ (See floor function)

*No Fermat number can be expressed as the sum of two primes, with the exception of F1 = 2 + 3.
*No Fermat prime can be expressed as the difference of two "p"th powers, where "p" is an odd prime.

* The sum of the reciprocals of all the Fermat numbers OEIS|id=A051158 is irrational. (Solomon W. Golomb, 1963)

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 "F"0,...,"F"4 are easily shown to be prime. However, this conjecture was refuted by Leonhard Euler in 1732 when he showed that

:$F_\left\{5\right\} = 2^\left\{2^5\right\} + 1 = 2^\left\{32\right\} + 1 = 4294967297 = 641 cdot 6700417. ;$

Euler proved that every factor of "F""n" must have the form "k"2"n"+1 + 1. For "n" = 5, this means that the only possible factors are of the form 64"k" + 1. Euler found the factor 641 = 10&times;64 + 1.

It is widely believedwho that Fermat was aware of the form of the factors later proved by Euler, 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 "F""n" with "n" > 4. However, little is known about Fermat numbers with large "n". [Chris Caldwell, [http://primes.utm.edu/links/theory/special_forms/ "Prime Links++: special forms"] at The Prime Pages.] In fact, each of the following is an open problem:

*Is "F""n" composite for all "n" > 4?
*Are there infinitely many Fermat primes? (Eisenstein 1844)
*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

:: $A sum_\left\{n=0\right\}^\left\{infty\right\} frac\left\{1\right\}\left\{ln F_\left\{n = frac\left\{A\right\}\left\{ln 2\right\} sum_\left\{n=0\right\}^\left\{infty\right\} frac\left\{1\right\}\left\{log_\left\{2\right\}\left(2^\left\{2^\left\{n+1\right)\right\} < frac\left\{A\right\}\left\{ln 2\right\} sum_\left\{n=0\right\}^\left\{infty\right\} 2^\left\{-n\right\} = frac\left\{2A\right\}\left\{ln 2\right\}.$

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.If (more sophisticatedly)we regard the conditional probability that "n" is prime, given that we know all its prime factors exceed "B", as at most "A"ln("B")/ln("n"), thenusing Euler's theorem that the least prime factor of "F""n"exceeds 2"n"+1, we would find instead

:: $A sum_\left\{n=0\right\}^\left\{infty\right\} frac\left\{ln 2^\left\{n+1\left\{ln F_\left\{n = A sum_\left\{n=0\right\}^\left\{infty\right\} frac\left\{log_2 2^\left\{n+1\left\{log_\left\{2\right\}\left(2^\left\{2^\left\{n+1\right)\right\} < A sum_\left\{n=0\right\}^\left\{infty\right\} \left(n+1\right) 2^\left\{-n\right\} = 4A.$

Although such arguments engender the belief that there are only finitely many Fermat primes, one can also produce arguments for the opposite conclusion.Suppose we regard the conditional probability that "n" is prime, given that we know all its prime factors are 1 modulo M,as at least "CM"/ln("n").Then using Euler's result that "M"=2"n"+1we would find that the expected total number of Fermat primes was at least

:: $C sum_\left\{n=0\right\}^\left\{infty\right\} frac\left\{2^\left\{n+1\left\{ln F_\left\{n = frac\left\{C\right\}\left\{ln 2\right\} sum_\left\{n=0\right\}^\left\{infty\right\} frac\left\{2^\left\{n+1\left\{log_\left\{2\right\}\left(2^\left\{2^\left\{n+1\right)\right\} > frac\left\{C\right\}\left\{ln 2\right\} sum_\left\{n=0\right\}^\left\{infty\right\} 1 = infty,$

and indeed this argument predicts that an asymptotically constant fraction of Fermat numbers are prime!

As of|2008 it is known that "F""n" is composite for 5 ≤ "n" ≤ 32, although complete factorizations of "F""n" are known only for 0 ≤ "n" ≤ 11, and there are no known factors for "n" in {14, 20, 22, 24}. The largest Fermat number known to be composite is "F"2478782, and its prime factor 3&times;22478785 + 1 was discovered by John B. Cosgrave and his Proth-Gallot Group on October 10 2003.

There are a number of conditions that are equivalent to the primality of "F""n".

*Proth's theorem -- (1878) Let "N" = "k"2"m" + 1 with odd "k" < 2"m". If there is an integer "a" such that

:: $a^\left\{\left(N-1\right)/2\right\} equiv -1 mod N$

:then "N" is prime. Conversely, if the above congruence does not hold, and in addition

:: $left\left(frac\left\{a\right\}\left\{N\right\} ight\right)=-1$ (See Jacobi symbol)

:then "N" is composite. If "N" = "F""n" > 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 Pépin's test. Although Pépin'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.
*Let "n" ≥ 3 be a positive odd integer. Then "n" is a Fermat prime if and only if for every "a" co-prime to "n", "a" is a primitive root mod "n" if and only if "a" is a quadratic nonresidue mod "n".
*The Fermat number "F""n" > 3 is prime if and only if it can be written uniquely as a sum of two nonzero squares, namely

:: $F_\left\{n\right\}=left\left(2^\left\{2^\left\{n-1 ight\right)^\left\{2\right\}+1^\left\{2\right\}$

:When $F_\left\{n\right\} = x^2 + y^2$ not of the form shown above, a proper factor is:

:: $gcd\left(x + 2^\left\{2^\left\{n-1 y, F_\left\{n\right\}\right)$

:Example 1: "F"5 = 622642 + 204492, so a proper factor is $gcd\left(62264, +, 2^\left\{2^4\right\}, 20449,, F_\left\{5\right\}\right) = 641$.

:Example 2: "F"6 = 40468032562 + 14387937592, so a proper factor is $gcd\left(4046803256, +, 2^\left\{2^5\right\}, 1438793759,, F_\left\{6\right\}\right) = 274177$.

Factorization of Fermat numbers

Because of the size of Fermat numbers, it is difficult to factorize or to prove primality of those. Pépin's test is a necessary and sufficient test for primality of Fermat numbers which can be implemented by modern computers. The elliptic curve method is a fast method for finding small prime divisors of numbers. Distributed computing project "Fermatsearch" has successfully found some factors of Fermat numbers. Yves Gallot's proth.exe has been used to find factors of large Fermat numbers. Edouard Lucas proved in 1878 that every factor of Fermat number $F_n$ is of the form $2^\left\{n+2\right\}k+1$, where k is a positive integer.

Pseudoprimes and Fermat numbers

Like composite numbers of the form 2"p" − 1, every composite Fermat number is a strong pseudoprime to base 2. Because "all" strong pseudoprimes to be 2 are also Fermat pseudoprimes - ie.

$2^\left\{F_n-1\right\} equiv 1 pmod\left\{F_n\right\},!$

for all Fermat numbers.

Because it is generally believed that all but the first few Fermat numbers are composite, this makes it possible to generate infinitely many strong pseudoprimes to base 2 from the Fermat numbers.

In fact, Rotkiewicz showed in 1964 that the product of any number of prime "or" composite Fermat numbers will be a Fermat pseudoprime to base 2.

Lemma: If "n" is a positive integer,

:$a^n-b^n=\left(a-b\right)sum_\left\{k=0\right\}^\left\{n-1\right\} a^kb^\left\{n-1-k\right\}.$

"proof:"

:$\left(a-b\right)sum_\left\{k=0\right\}^\left\{n-1\right\}a^kb^\left\{n-1-k\right\}$

:$=sum_\left\{k=0\right\}^\left\{n-1\right\}a^\left\{k+1\right\}b^\left\{n-1-k\right\}-sum_\left\{k=0\right\}^\left\{n-1\right\}a^kb^\left\{n-k\right\}$

:$=a^n+sum_\left\{k=1\right\}^\left\{n-1\right\}a^kb^\left\{n-k\right\}-sum_\left\{k=1\right\}^\left\{n-1\right\}a^kb^\left\{n-k\right\}-b^n$

:$=a^n-b^n$

Theorem: If $2^n+1$ is prime, then $n$ is zero or a power of 2.

"proof:"

For $n=0$, $2^0+1$ equals prime number 2. (This is why some sources count 2 as a sixth Fermat prime.)

If $n$ is a positive integer but not a power of 2, then $n = rs$ where $1 le r < n$, $1 < s le n$ and $s$ is odd.

By the preceding lemma, for positive integer $m$,

:$\left(a-b\right) mid \left(a^m-b^m\right)$

where $mid$ means "evenly divides". Substituting $a = 2^r$, $b = -1$, and $m = s$,

:$\left(2^r+1\right) mid \left(2^\left\{rs\right\}+1\right),$and thus:$\left(2^r+1\right) mid \left(2^n+1\right).$

Because $2^r+1 > 1$, $2^n+1$ is not prime when $n$ is a positive integer that is not a power of 2.

A theorem of Édouard Lucas: Any prime divisor "p" of "F"n = $2^\left\{2^\left\{overset\left\{n\right\}\left\{\right\}+1$ is of the form $k2^\left\{n+2\right\}+1$ whenever n is greater than one.

"sketch of proof:"

Let "G""p" denote the group of non-zero elements of the integers (mod "p") under multiplication, which has order "p-1". Notice that "2" (strictly speaking, its image (mod "p")) has multiplicative order $2^\left\{n+1\right\}$ in "G""p", so that, by Lagrange's theorem, "p-1" is divisible by $2^\left\{n+1\right\}$ and "p" has the form $k2^\left\{n+1\right\}+1$ for some integer "k",as Euler knew. Édouard Lucas went further. Since "n" is greater than "1", the prime "p" above is congruent to 1 (mod "8"). Hence (as was known to Carl Friedrich Gauss), "2" is a quadratic residue (mod "p"), that is, there in integer "a" such that "a"2 -2 is divisible by "p". Then the image of "a" has order $2^\left\{n+2\right\}$ in the group "G""p" and (using Lagrange's theorem again), "p-1" is divisible by $2^\left\{n+2\right\}$and "p" has the form $s2^\left\{n+2\right\}+1$ for some integer "s".

In fact, it can be seen directly that "2" is a quadratic residue (mod "p"), since $\left(1 +2^\left\{2^\left\{n-1\right)^\left\{2\right\} equiv 2^\left\{1+2^\left\{n-1$ (mod "p"). Since anodd power of "2" is a quadratic residue (mod "p"), so is "2" itself.

Relationship to constructible polygons

An "n"-sided regular polygon can be constructed with compass and straightedge if and only if "n" is the product of a power of 2 and distinct Fermat primes. In other words, if and only if "n" is of the form "n" = 2"k""p"1"p"2…"p""s", where "k" is a nonnegative integer and the "p""i" 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

Pseudorandom Number Generation

Fermat primes are particularly useful in generating pseudo-random sequences of numbers in the range 1 … "N", where "N" is a power of 2. The most common method used is to take any seed value between 1 and "P" − 1, where "P" is a Fermat prime. Now multiply this by a number "A", which is greater than the square root of "P" and is a primitive root modulo "P" (i.e., it is not a quadratic residue). Then take the result modulo "P". The result is the new value for the RNG.: (see Linear congruential generator, RANDU)This is useful in computer science since most data structures have members with 2"X" possible values. For example, a byte has 256 (28) possible values (0–255). Therefore to fill a byte or bytes with random values a random number generator which produces values 1–256 can be used, the byte taking the output value − 1. Very large Fermat primes are of particular interest in data encryption for this reason. This method produces only pseudorandom values as, after "P" − 1 repetitions, the sequence repeats. A poorly chosen multiplier can result in the sequence repeating sooner than "P" − 1.

Other interesting facts

A Fermat number cannot be a perfect number or part of a pair of amicable numbers.(Luca 2000)

The series of reciprocals of all prime divisors of Fermat numbers is convergent.(Krizek, Luca, Somer 2002)

If "n""n" + 1 is prime, there exists an integer "m" such that "n" = 22"m". The equation"n""n" + 1 = "F"(2"m"+"m")holds at that time. [Jeppe Stig Nielsen, [http://jeppesn.dk/nton.html "S(n) = n^n + 1"] .]

Let the largest prime factor of Fermat number "F""n" be "P"("F""n"). Then,:$P\left(F_n \right)ge 2^\left\{m+2\right\}\left(4m+9\right)+1.$ (Grytczuk, Luca and Wojtowicz, 2001）

A Fermat prime cannot also be a Wieferich prime. (Luca)

Generalized Fermat numbers

Numbers of the form $a^\left\{2^\left\{ overset\left\{n\right\} \left\{\right\} + b^\left\{2^\left\{ overset\left\{n\right\} \left\{\right\}$, where "a" > 1 are called generalized Fermat numbers. By analogy with the ordinary Fermat numbers, it is common to write generalized Fermat numbers of the form $a^\left\{2^\left\{ overset\left\{n\right\} \left\{\right\} + 1$ as "Fn"("a"). In this notation, for instance, the number 100,000,001 would be written as "F"3(10)

An odd prime "p" is a generalized Fermat number if and only if "p" is congruent to 1 ( mod 4).

Generalized Fermat primes

Because of the ease of proving their primality, generalized Fermat primes have become in recent years a hot topic for research within the field of number theory. Many of the largest known primes today are generalized Fermat primes.

Generalized Fermat numbers can be prime only for even "a", because if a is odd then every generalized Fermat number will be divisible by 2. By analogy with the heuristic argument for the finite number of primes among the base-2 Fermat numbers, it is to be expected that there will be only finitely many generalized Fermat primes for each even base. The smallest prime number "Fn"("a") with "n" > 4 is "F"5(30), or 3032+1.

A more elaborate theory can be used to predict the number of bases for which "Fn"("a") will be prime for a fixed "n". The number of generalized Fermat primes can be roughly expected to halve as "n" is increased by 1.

ee also

* Mersenne prime
* Lucas' theorem
* Proth's theorem
* Pseudoprime
* Primality test
* Constructible number
* Sierpiński number
* Sylvester's sequence
* Double exponential function

Notes

References

* "17 Lectures on Fermat Numbers: From Number Theory to Geometry", Michal Křížek, Florian Luca, Lawrence Somer, Springer, CMS Books 9, ISBN 0-387-95332-9 (This book contains an extensive list of references.)
*S. W. Golomb, On the sum of the reciprocals of the Fermat numbers and related irrationalities, Canad. J. Math. 15(1963), 475–478.
* Richard K. Guy, "Unsolved Problems in Number Theory" (3rd ed), Springer Verlag, 2004 ISBN 0-387-20860-7; sections A3, A12, B21.
*Florian Luca, The anti-social Fermat number, Amer. Math. Monthly 107(2000), 171–173.
*Michal Křížek, Florian Luca and Lawrence Somer(2002), On the convergence of series of reciprocals of primes related to the Fermat numbers, J. Number Theory 97(2002), 95–112.
* A. Grytczuk, F. Luca and M. Wojtowicz(2001), Another note on the greatest prime factors of Fermat numbers, "Southeast Asian Bull. Math." 25(2001), 111–115.

* [http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=A000215 Sequence of Fermat numbers] at OEIS.
* Chris Caldwell, [http://primes.utm.edu/glossary/page.php?sort=FermatNumber The Prime Glossary: Fermat number] at The Prime Pages.
* Luigi Morelli, [http://www.fermatsearch.org/history.html History of Fermat Numbers]
* John Cosgrave, [http://www.spd.dcu.ie/johnbcos/fermat6.htm Unification of Mersenne and Fermat Numbers]
* Wilfrid Keller, [http://www.prothsearch.net/fermat.html Prime Factors of Fermat Numbers]
*
* Yves Gallot, [http://pagesperso-orange.fr/yves.gallot/primes/index.html Generalized Fermat Prime Search]
* Mark S. Manasse, [http://www.google.com/groups?selm=1990Jun15.190100.8505%40src.dec.com&oe=UTF-8&output=gplain Complete factorization of the ninth Fermat number] (original announcement)

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