# Fermat number

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

Named after | Pierre de Fermat |
---|---|

No. of known terms | 5 |

Conjectured no. of terms | 5 |

Subsequence of | Fermat numbers |

First terms | 3, 5, 17, 257, 65537 |

Largest known term | 65537 |

OEIS index | A019434 |

where *n* is a nonnegative integer. The first few Fermat numbers are:

If 2^{k} + 1 is prime, and *k* > 0, it can be shown that *k* must be a power of two. (If *k* = *ab* where 1 ≤ *a*, *b* ≤ *k* and *b* is odd, then 2^{k} + 1 = (2^{a})^{b} + 1 ≡ (−1)^{b} + 1 = 0 (**mod** 2^{a} + 1). See below for a complete proof.) In other words, every prime of the form 2^{k} + 1 (other than 2 = 2^{0} + 1) is a Fermat number, and such primes are called **Fermat primes**. As of 2019, the only known Fermat primes are *F*_{0}, *F*_{1}, *F*_{2}, *F*_{3}, and *F*_{4} (sequence A019434 in the OEIS).

## Basic properties

The Fermat numbers satisfy the following recurrence relations:

for *n* ≥ 1,

for *n* ≥ 2. Each of these relations can be proved by mathematical induction. From the last equation, we can deduce **Goldbach's theorem** (named after Christian Goldbach): no two Fermat numbers share a common integer factor greater than 1. To see this, suppose that 0 ≤ *i* < *j* and *F*_{i} and *F*_{j} have a common factor *a* > 1. Then *a* divides both

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

- No Fermat prime can be expressed as the difference of two
*p*th powers, where*p*is an odd prime. - With the exception of F
_{0}and F_{1}, the last digit of a Fermat number is 7. - The sum of the reciprocals of all the Fermat numbers (sequence A051158 in the OEIS) is irrational. (Solomon W. Golomb, 1963)

## Primality of Fermat numbers

Fermat numbers and Fermat primes were first studied by Pierre de Fermat, who conjectured (but admitted he could not prove) that all Fermat numbers are prime. Indeed, the first five Fermat numbers *F*_{0},...,*F*_{4} are easily shown to be prime. However, the conjecture was refuted by Leonhard Euler in 1732 when he showed that

Euler proved that every factor of *F*_{n} must have the form *k*2^{n+1} + 1 (later improved to *k*2^{n+2} + 1 by Lucas).

The fact that 641 is a factor of *F*_{5} can be deduced from the equalities 641 = 2^{7} × 5 + 1 and 641 = 2^{4} + 5^{4}. It follows from the first equality that 2^{7} × 5 ≡ −1 (mod 641) and therefore (raising to the fourth power) that 2^{28} × 5^{4} ≡ 1 (mod 641). On the other hand, the second equality implies that 5^{4} ≡ −2^{4} (mod 641). These congruences imply that −2^{32} ≡ 1 (mod 641).

Fermat was probably 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.[1] One common explanation is that Fermat made a computational mistake.

There are no other known Fermat primes *F*_{n} with *n* > 4. However, little is known about Fermat numbers with large *n*.[2] 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)[3]
- Are there infinitely many composite Fermat numbers?
- Does a Fermat number exist that is not square-free?

As of 2014, it is known that *F*_{n} is composite for 5 ≤ *n* ≤ 32, although amongst these, complete factorizations of *F*_{n} are known only for 0 ≤ *n* ≤ 11, and there are no known prime factors for *n* = 20 and *n* = 24.[4] The largest Fermat number known to be composite is *F*_{3329780}, and its prime factor 193 × 2^{3329782} + 1, a megaprime, was discovered by the PrimeGrid collaboration in July 2014.[4][5]

### Heuristic arguments for density

There are several probabilistic arguments for the finitude of Fermat primes.

According to the prime number theorem, the "probability" that a number *n* is prime is about 1/ln(*n*). Therefore, the total expected number of Fermat primes is at most

This argument is not 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*), then using Euler's theorem that the least prime factor of *F*_{n} exceeds 2^{n + 1}, we would find instead

### Equivalent conditions of primality

Let be the *n*th Fermat number. Pépin's test states that for *n* > 0,

- is prime if and only if

The expression can be evaluated modulo by repeated squaring. This makes the test a fast polynomial-time algorithm. However, Fermat numbers grow so rapidly that only a handful of Fermat numbers can be tested in a reasonable amount of time and space.

There are some tests that can be used to test numbers of the form *k*2^{m} + 1, such as factors of Fermat numbers, for primality.

**Proth's theorem**(1878). Let*N*=*k*2^{m}+ 1 with odd*k*< 2^{m}. If there is an integer*a*such that- then
*N*is prime. Conversely, if the above congruence does not hold, and in addition- (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 some Fermat numbers, neither test gives a specific nontrivial factor. In fact, no specific prime factors are known for *n* = 20 and 24.

## Factorization of Fermat numbers

Because of the size of Fermat numbers, it is difficult to factorize or even to check primality. Pépin's test gives a necessary and sufficient condition for primality of Fermat numbers, and 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. Édouard Lucas, improving the above-mentioned result by Euler, proved in 1878 that every factor of Fermat number , with *n* at least 2, is of the form (see Proth number), where *k* is a positive integer. By itself, this makes it easy to prove the primality of the known Fermat primes.

Factorizations of the first twelve Fermat numbers are:

*F*_{0}= 2 ^{1}+ 1 = 3 is prime *F*_{1}= 2 ^{2}+ 1 = 5 is prime *F*_{2}= 2 ^{4}+ 1 = 17 is prime *F*_{3}= 2 ^{8}+ 1 = 257 is prime *F*_{4}= 2 ^{16}+ 1 = 65,537 is the largest known Fermat prime *F*_{5}= 2 ^{32}+ 1 = 4,294,967,297 = 641 × 6,700,417 (fully factored 1732) *F*_{6}= 2 ^{64}+ 1 = 18,446,744,073,709,551,617 (20 digits) = 274,177 × 67,280,421,310,721 (14 digits) (fully factored 1855) *F*_{7}= 2 ^{128}+ 1 = 340,282,366,920,938,463,463,374,607,431,768,211,457 (39 digits) = 59,649,589,127,497,217 (17 digits) × 5,704,689,200,685,129,054,721 (22 digits) (fully factored 1970) *F*_{8}= 2 ^{256}+ 1 = 115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457,584,007,913,129,

639,937 (78 digits)= 1,238,926,361,552,897 (16 digits) ×

93,461,639,715,357,977,769,163,558,199,606,896,584,051,237,541,638,188,580,280,321 (62 digits) (fully factored 1980)*F*_{9}= 2 ^{512}+ 1 = 13,407,807,929,942,597,099,574,024,998,205,846,127,479,365,820,592,393,377,723,561,443,721,764,0

30,073,546,976,801,874,298,166,903,427,690,031,858,186,486,050,853,753,882,811,946,569,946,433,6

49,006,084,097 (155 digits)= 2,424,833 × 7,455,602,825,647,884,208,337,395,736,200,454,918,783,366,342,657 (49 digits) ×

741,640,062,627,530,801,524,787,141,901,937,474,059,940,781,097,519,023,905,821,316,144,415,759,

504,705,008,092,818,711,693,940,737 (99 digits) (fully factored 1990)*F*_{10}= 2 ^{1024}+ 1 = 179,769,313,486,231,590,772,930...304,835,356,329,624,224,137,217 (309 digits) = 45,592,577 × 6,487,031,809 × 4,659,775,785,220,018,543,264,560,743,076,778,192,897 (40 digits) ×

130,439,874,405,488,189,727,484...806,217,820,753,127,014,424,577 (252 digits) (fully factored 1995)*F*_{11}= 2 ^{2048}+ 1 = 32,317,006,071,311,007,300,714,8...193,555,853,611,059,596,230,657 (617 digits) = 319,489 × 974,849 × 167,988,556,341,760,475,137 (21 digits) × 3,560,841,906,445,833,920,513 (22 digits) ×

173,462,447,179,147,555,430,258...491,382,441,723,306,598,834,177 (564 digits) (fully factored 1988)

As of 2018, only *F*_{0} to *F*_{11} have been completely factored.[4] The distributed computing project Fermat Search is searching for new factors of Fermat numbers.[6] The set of all Fermat factors is A050922 (or, sorted, A023394) in OEIS.

It is possible that the only primes of this form are 3, 5, 17, 257 and 65,537. Indeed, Boklan and Conway published in 2016 a very precise analysis suggesting that the probability of the existence of another Fermat prime is less than one in a billion.[7]

The following factors of Fermat numbers were known before 1950 (since the '50s, digital computers have helped find more factors):

Year | Finder | Fermat number | Factor |
---|---|---|---|

1732 | Euler | ||

1732 | Euler | (fully factored) | |

1855 | Clausen | ||

1855 | Clausen | (fully factored) | |

1877 | Pervushin | ||

1878 | Pervushin | ||

1886 | Seelhoff | ||

1899 | Cunningham | ||

1899 | Cunningham | ||

1903 | Western | ||

1903 | Western | ||

1903 | Western | ||

1903 | Western | ||

1903 | Cullen | ||

1906 | Morehead | ||

1925 | Kraitchik | ||

As of March 2019, 349 prime factors of Fermat numbers are known, and 305 Fermat numbers are known to be composite.[4] Several new Fermat factors are found each year.[8]

## Pseudoprimes and Fermat numbers

Like composite numbers of the form 2^{p} − 1, every composite Fermat number is a strong pseudoprime to base 2. This is because all strong pseudoprimes to base 2 are also Fermat pseudoprimes - i.e.

for all Fermat numbers.

In 1904, Cipolla showed that the product of at least two distinct prime or composite Fermat numbers will be a Fermat pseudoprime to base 2 if and only if .[9]

## Other theorems about Fermat numbers

**Lemma.** — If *n* is a positive integer,

**Proof**

**Theorem** — If is an odd prime, then is a power of 2.

**Proof**

If is a positive integer but not a power of 2, it must have an odd prime factor , and we may write where .

By the preceding lemma, for positive integer ,

where means "evenly divides". Substituting , and and using that is odd,

and thus

Because , it follows that is not prime. Therefore, by contraposition must be a power of 2.

**Theorem** — A Fermat prime cannot be a Wieferich prime.

**Proof**

We show if is a Fermat prime (and hence by the above, *m* is a power of 2), then the congruence does not hold.

Since we may write . If the given congruence holds, then , and therefore

Hence , and therefore . This leads to , which is impossible since .

**Theorem** (Édouard Lucas) — Any prime divisor *p* of is of the form whenever *n* > 1.

*Sketch of proof*Let *G*_{p} denote the group of non-zero elements of the integers modulo *p* under multiplication, which has order *p-1*. Notice that 2 (strictly speaking, its image modulo *p*) has multiplicative order dividing in *G*_{p} (since is the square of which is −1 modulo *F _{n}*), so that, by Lagrange's theorem,

*p*− 1 is divisible by and

*p*has the form for some integer

*k*, as Euler knew. Édouard Lucas went further. Since

*n*> 1, the prime

*p*above is congruent to 1 modulo 8. Hence (as was known to Carl Friedrich Gauss), 2 is a quadratic residue modulo

*p*, that is, there is integer

*a*such that Then the image of

*a*has order in the group

*G*

_{p}and (using Lagrange's theorem again),

*p*− 1 is divisible by and

*p*has the form for some integer

*s*.

In fact, it can be seen directly that 2 is a quadratic residue modulo *p*, since

Since an odd power of 2 is a quadratic residue modulo *p*, so is 2 itself.

## Relationship to constructible polygons

Carl Friedrich Gauss developed the theory of Gaussian periods in his *Disquisitiones Arithmeticae* and formulated a sufficient condition for the constructibility of regular polygons. Gauss stated without proof that this condition was also necessary, but never published his proof. A full proof of necessity was given by Pierre Wantzel in 1837. The result is known as the **Gauss–Wantzel theorem**:

- 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.

A positive integer *n* is of the above form if and only if its totient φ(*n*) is a power of 2.

## 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.

This is useful in computer science since most data structures have members with 2^{X} possible values. For example, a byte has 256 (2^{8}) 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. (Křížek, Luca & Somer 2002)

If *n*^{n} + 1 is prime, there exists an integer *m* such that *n* = 2^{2}^{m}. The equation
*n*^{n} + 1 = *F*_{(2}_{m}_{+m)}
holds in that case.[10][11]

Let the largest prime factor of Fermat number *F*_{n} be *P*(*F*_{n}). Then,

## Generalized Fermat numbers

Numbers of the form with *a*, *b* any coprime integers, *a* > *b* > 0, are called **generalized Fermat numbers**. An odd prime *p* is a generalized Fermat number if and only if *p* is congruent to 1 (mod 4). (Here we consider only the case *n* > 0, so 3 = is not a counterexample.)

An example of a probable prime of this form is 124^{65536} + 57^{65536} (found by Valeryi Kuryshev).[12]

By analogy with the ordinary Fermat numbers, it is common to write generalized Fermat numbers of the form as *F _{n}*(

*a*). In this notation, for instance, the number 100,000,001 would be written as

*F*

_{3}(10). In the following we shall restrict ourselves to primes of this form, , such primes are called "Fermat primes base

*a*". Of course, these primes exist only if

*a*is even.

If we require *n* > 0, then Landau's fourth problem asks if there are infinitely many generalized Fermat primes *F _{n}*(

*a*).

### Generalized Fermat primes

Because of the ease of proving their primality, generalized Fermat primes have become in recent years a 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. The smallest prime number with is , or 30^{32}+1. Besides, we can define "half generalized Fermat numbers" for an odd base, a half generalized Fermat number to base *a* (for odd *a*) is , and it is also to be expected that there will be only finitely many half generalized Fermat primes for each odd base.

(In the list, the generalized Fermat numbers () to an even a are , for odd a, they are . If a is a perfect power with an odd exponent (sequence A070265 in the OEIS), then all generalized Fermat number can be algebraic factored, so they cannot be prime)

(For the smallest number such that is prime, see OEIS: A253242)

numbers such that is prime |
numbers such that is prime |
numbers such that is prime |
numbers such that is prime | ||||
---|---|---|---|---|---|---|---|

2 | 0, 1, 2, 3, 4, ... | 18 | 0, ... | 34 | 2, ... | 50 | ... |

3 | 0, 1, 2, 4, 5, 6, ... | 19 | 1, ... | 35 | 1, 2, 6, ... | 51 | 1, 3, 6, ... |

4 | 0, 1, 2, 3, ... | 20 | 1, 2, ... | 36 | 0, 1, ... | 52 | 0, ... |

5 | 0, 1, 2, ... | 21 | 0, 2, 5, ... | 37 | 0, ... | 53 | 3, ... |

6 | 0, 1, 2, ... | 22 | 0, ... | 38 | ... | 54 | 1, 2, 5, ... |

7 | 2, ... | 23 | 2, ... | 39 | 1, 2, ... | 55 | ... |

8 | (none) | 24 | 1, 2, ... | 40 | 0, 1, ... | 56 | 1, 2, ... |

9 | 0, 1, 3, 4, 5, ... | 25 | 0, 1, ... | 41 | 4, ... | 57 | 0, 2, ... |

10 | 0, 1, ... | 26 | 1, ... | 42 | 0, ... | 58 | 0, ... |

11 | 1, 2, ... | 27 | (none) | 43 | 3, ... | 59 | 1, ... |

12 | 0, ... | 28 | 0, 2, ... | 44 | 4, ... | 60 | 0, ... |

13 | 0, 2, 3, ... | 29 | 1, 2, 4, ... | 45 | 0, 1, ... | 61 | 0, 1, 2, ... |

14 | 1, ... | 30 | 0, 5, ... | 46 | 0, 2, 9, ... | 62 | ... |

15 | 1, ... | 31 | ... | 47 | 3, ... | 63 | ... |

16 | 0, 1, 2, ... | 32 | (none) | 48 | 2, ... | 64 | (none) |

17 | 2, ... | 33 | 0, 3, ... | 49 | 1, ... | 65 | 1, 2, 5, ... |

b |
known generalized (half) Fermat prime base b |

2 | 3, 5, 17, 257, 65537 |

3 | 2, 5, 41, 21523361, 926510094425921, 1716841910146256242328924544641 |

4 | 5, 17, 257, 65537 |

5 | 3, 13, 313 |

6 | 7, 37, 1297 |

7 | 1201 |

8 | (not possible) |

9 | 5, 41, 21523361, 926510094425921, 1716841910146256242328924544641 |

10 | 11, 101 |

11 | 61, 7321 |

12 | 13 |

13 | 7, 14281, 407865361 |

14 | 197 |

15 | 113 |

16 | 17, 257, 65537 |

17 | 41761 |

18 | 19 |

19 | 181 |

20 | 401, 160001 |

21 | 11, 97241, 1023263388750334684164671319051311082339521 |

22 | 23 |

23 | 139921 |

24 | 577, 331777 |

25 | 13, 313 |

26 | 677 |

27 | (not possible) |

28 | 29, 614657 |

29 | 421, 353641, 125123236840173674393761 |

30 | 31, 185302018885184100000000000000000000000000000001 |

31 | |

32 | (not possible) |

33 | 17, 703204309121 |

34 | 1336337 |

35 | 613, 750313, 330616742651687834074918381127337110499579842147487712949050636668246738736343104392290115356445313 |

36 | 37, 1297 |

37 | 19 |

38 | |

39 | 761, 1156721 |

40 | 41, 1601 |

41 | 31879515457326527173216321 |

42 | 43 |

43 | 5844100138801 |

44 | 197352587024076973231046657 |

45 | 23, 1013 |

46 | 47, 4477457, 46^{512}+1 (852 digits: 214787904487...289480994817) |

47 | 11905643330881 |

48 | 5308417 |

49 | 1201 |

50 |

(See [13][14] for more information (even bases up to 1000), also see [15] for odd bases)

(For the smallest prime of the form (for odd ), see also OEIS: A111635)

numbers such that is prime | ||
---|---|---|

2 | 1 | 0, 1, 2, 3, 4, ... |

3 | 1 | 0, 1, 2, 4, 5, 6, ... |

3 | 2 | 0, 1, 2, ... |

4 | 1 | 0, 1, 2, 3, ... |

4 | 3 | 0, 2, 4, ... |

5 | 1 | 0, 1, 2, ... |

5 | 2 | 0, 1, 2, ... |

5 | 3 | 1, 2, 3, ... |

5 | 4 | 1, 2, ... |

6 | 1 | 0, 1, 2, ... |

6 | 5 | 0, 1, 3, 4, ... |

7 | 1 | 2, ... |

7 | 2 | 1, 2, ... |

7 | 3 | 0, 1, 8, ... |

7 | 4 | 0, 2, ... |

7 | 5 | 1, 4, ... |

7 | 6 | 0, 2, 4, ... |

8 | 1 | (none) |

8 | 3 | 0, 1, 2, ... |

8 | 5 | 0, 1, 2, ... |

8 | 7 | 1, 4, ... |

9 | 1 | 0, 1, 3, 4, 5, ... |

9 | 2 | 0, 2, ... |

9 | 4 | 0, 1, ... |

9 | 5 | 0, 1, 2, ... |

9 | 7 | 2, ... |

9 | 8 | 0, 2, 5, ... |

10 | 1 | 0, 1, ... |

10 | 3 | 0, 1, 3, ... |

10 | 7 | 0, 1, 2, ... |

10 | 9 | 0, 1, 2, ... |

11 | 1 | 1, 2, ... |

11 | 2 | 0, 2, ... |

11 | 3 | 0, 3, ... |

11 | 4 | 1, 2, ... |

11 | 5 | 1, ... |

11 | 6 | 0, 1, 2, ... |

11 | 7 | 2, 4, 5, ... |

11 | 8 | 0, 6, ... |

11 | 9 | 1, 2, ... |

11 | 10 | 5, ... |

12 | 1 | 0, ... |

12 | 5 | 0, 4, ... |

12 | 7 | 0, 1, 3, ... |

12 | 11 | 0, ... |

13 | 1 | 0, 2, 3, ... |

13 | 2 | 1, 3, 9, ... |

13 | 3 | 1, 2, ... |

13 | 4 | 0, 2, ... |

13 | 5 | 1, 2, 4, ... |

13 | 6 | 0, 6, ... |

13 | 7 | 1, ... |

13 | 8 | 1, 3, 4, ... |

13 | 9 | 0, 3, ... |

13 | 10 | 0, 1, 2, 4, ... |

13 | 11 | 2, ... |

13 | 12 | 1, 2, 5, ... |

14 | 1 | 1, ... |

14 | 3 | 0, 3, ... |

14 | 5 | 0, 2, 4, 8, ... |

14 | 9 | 0, 1, 8, ... |

14 | 11 | 1, ... |

14 | 13 | 2, ... |

15 | 1 | 1, ... |

15 | 2 | 0, 1, ... |

15 | 4 | 0, 1, ... |

15 | 7 | 0, 1, 2, ... |

15 | 8 | 0, 2, 3, ... |

15 | 11 | 0, 1, 2, ... |

15 | 13 | 1, 4, ... |

15 | 14 | 0, 1, 2, 4, ... |

16 | 1 | 0, 1, 2, ... |

16 | 3 | 0, 2, 8, ... |

16 | 5 | 1, 2, ... |

16 | 7 | 0, 6, ... |

16 | 9 | 1, 3, ... |

16 | 11 | 2, 4, ... |

16 | 13 | 0, 3, ... |

16 | 15 | 0, ... |

(For the smallest even base a such that is prime, see OEIS: A056993)

bases a such that is prime (only consider even a) | OEIS sequence | |
---|---|---|

0 | 2, 4, 6, 10, 12, 16, 18, 22, 28, 30, 36, 40, 42, 46, 52, 58, 60, 66, 70, 72, 78, 82, 88, 96, 100, 102, 106, 108, 112, 126, 130, 136, 138, 148, 150, ... | A006093 |

1 | 2, 4, 6, 10, 14, 16, 20, 24, 26, 36, 40, 54, 56, 66, 74, 84, 90, 94, 110, 116, 120, 124, 126, 130, 134, 146, 150, 156, 160, 170, 176, 180, 184, ... | A005574 |

2 | 2, 4, 6, 16, 20, 24, 28, 34, 46, 48, 54, 56, 74, 80, 82, 88, 90, 106, 118, 132, 140, 142, 154, 160, 164, 174, 180, 194, 198, 204, 210, 220, 228, ... | A000068 |

3 | 2, 4, 118, 132, 140, 152, 208, 240, 242, 288, 290, 306, 378, 392, 426, 434, 442, 508, 510, 540, 542, 562, 596, 610, 664, 680, 682, 732, 782, ... | A006314 |

4 | 2, 44, 74, 76, 94, 156, 158, 176, 188, 198, 248, 288, 306, 318, 330, 348, 370, 382, 396, 452, 456, 470, 474, 476, 478, 560, 568, 598, 642, ... | A006313 |

5 | 30, 54, 96, 112, 114, 132, 156, 332, 342, 360, 376, 428, 430, 432, 448, 562, 588, 726, 738, 804, 850, 884, 1068, 1142, 1198, 1306, 1540, 1568, ... | A006315 |

6 | 102, 162, 274, 300, 412, 562, 592, 728, 1084, 1094, 1108, 1120, 1200, 1558, 1566, 1630, 1804, 1876, 2094, 2162, 2164, 2238, 2336, 2388, ... | A006316 |

7 | 120, 190, 234, 506, 532, 548, 960, 1738, 1786, 2884, 3000, 3420, 3476, 3658, 4258, 5788, 6080, 6562, 6750, 7692, 8296, 9108, 9356, 9582, ... | A056994 |

8 | 278, 614, 892, 898, 1348, 1494, 1574, 1938, 2116, 2122, 2278, 2762, 3434, 4094, 4204, 4728, 5712, 5744, 6066, 6508, 6930, 7022, 7332, ... | A056995 |

9 | 46, 1036, 1318, 1342, 2472, 2926, 3154, 3878, 4386, 4464, 4474, 4482, 4616, 4688, 5374, 5698, 5716, 5770, 6268, 6386, 6682, 7388, 7992, ... | A057465 |

10 | 824, 1476, 1632, 2462, 2484, 2520, 3064, 3402, 3820, 4026, 6640, 7026, 7158, 9070, 12202, 12548, 12994, 13042, 15358, 17646, 17670, ... | A057002 |

11 | 150, 2558, 4650, 4772, 11272, 13236, 15048, 23302, 26946, 29504, 31614, 33308, 35054, 36702, 37062, 39020, 39056, 43738, 44174, 45654, ... | A088361 |

12 | 1534, 7316, 17582, 18224, 28234, 34954, 41336, 48824, 51558, 51914, 57394, 61686, 62060, 89762, 96632, 98242, 100540, 101578, 109696, ... | A088362 |

13 | 30406, 71852, 85654, 111850, 126308, 134492, 144642, 147942, 150152, 165894, 176206, 180924, 201170, 212724, 222764, 225174, 241600, ... | A226528 |

14 | 67234, 101830, 114024, 133858, 162192, 165306, 210714, 216968, 229310, 232798, 422666, 426690, 449732, 462470, 468144, 498904, 506664, ... | A226529 |

15 | 70906, 167176, 204462, 249830, 321164, 330716, 332554, 429370, 499310, 524552, 553602, 743788, 825324, 831648, 855124, 999236, 1041870, ... | A226530 |

16 | 48594, 108368, 141146, 189590, 255694, 291726, 292550, 357868, 440846, 544118, 549868, 671600, 843832, 857678, 1024390, 1057476, 1087540, ... | A251597 |

17 | 62722, 130816, 228188, 386892, 572186, 689186, 909548, 1063730, 1176694, 1361244, 1372930, 1560730, 1660830, 1717162, 1722230, 1766192, ... | A253854 |

18 | 24518, 40734, 145310, 361658, 525094, 676754, 773620, 1415198, 1488256, 1615588, 1828858, 2042774, 2514168, 2611294, 2676404, 3060772, ... | A244150 |

19 | 75898, 341112, 356926, 475856, 1880370, 2061748, 2312092, ... | A243959 |

20 | 919444, 1059094, ... | A321323 |

The smallest base *b* such that *b*^{2n} + 1 is prime are

- 2, 2, 2, 2, 2, 30, 102, 120, 278, 46, 824, 150, 1534, 30406, 67234, 70906, 48594, 62722, 24518, 75898, 919444, ... (sequence A056993 in the OEIS)

The smallest *k* such that (2*n*)^{k} + 1 is prime are

- 1, 1, 1, 0, 1, 1, 2, 1, 1, 2, 1, 2, 2, 1, 1, 0, 4, 1, ... (The next term is unknown) (sequence A079706 in the OEIS) (also see OEIS: A228101 and OEIS: A084712)

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

### Largest known generalized Fermat primes

The following is a list of the 5 largest known generalized Fermat primes.[16] They are all megaprimes. As of September 2018 the whole top-5 was discovered by participants in the PrimeGrid project.

Rank | Prime rank[17] | Prime number | Generalized Fermat notation | Number of digits | Found date | ref. |
---|---|---|---|---|---|---|

1 | 14 | 1059094^{1048576} + 1 |
F_{20}(1059094) |
6,317,602 | Nov 2018 | [18] |

2 | 15 | 919444^{1048576} + 1 |
F_{20}(919444) |
6,253,210 | Sep 2017 | [19] |

3 | 26 | 2877652^{524288} + 1 |
F_{19}(2877652) |
3,386,397 | Jun 2019 | [20] |

4 | 27 | 2788032^{524288} + 1 |
F_{19}(2788032) |
3,379,193 | Apr 2019 | [21] |

5 | 28 | 2733014^{524288} + 1 |
F_{19}(2733014) |
3,374,655 | Mar 2019 | [22] |

On the Prime Pages you can perform a search yielding the current top 100 generalized Fermat primes.

## See also

- Constructible polygon: which regular polygons are constructible partially depends on Fermat primes.
- Double exponential function
- Lucas' theorem
- Mersenne prime
- Pierpont prime
- Primality test
- Proth's theorem
- Pseudoprime
- Sierpiński number
- Sylvester's sequence

## Notes

- Křížek, Luca & Somer 2001, p. 38, Remark 4.15
- Chris Caldwell, "Prime Links++: special forms" Archived 2013-12-24 at the Wayback Machine at The Prime Pages.
- Ribenboim 1996, p. 88.
- Keller, Wilfrid (February 7, 2012), "Prime Factors of Fermat Numbers",
*ProthSearch.com*, retrieved January 14, 2017 - "PrimeGrid's Mega Prime Search – 193*2^3329782+1 (official announcement)" (PDF). PrimeGrid. Retrieved 7 August 2014.
- ":: F E R M A T S E A R C H . O R G :: Home page".
*www.fermatsearch.org*. Retrieved 7 April 2018. - Boklan, Kent D.; Conway, John H. (2016). "Expect at most one billionth of a new Fermat Prime!". arXiv:1605.01371 [math.NT].
- ":: F E R M A T S E A R C H . O R G :: News".
*www.fermatsearch.org*. Retrieved 7 April 2018. - Krizek, Michal; Luca, Florian; Somer, Lawrence (14 March 2013).
*17 Lectures on Fermat Numbers: From Number Theory to Geometry*. Springer Science & Business Media. ISBN 9780387218502. Retrieved 7 April 2018 – via Google Books. - Jeppe Stig Nielsen, "S(n) = n^n + 1".
- Weisstein, Eric W. "Sierpiński Number of the First Kind".
*MathWorld*. - PRP Top Records, search for x^(2^16)+y^(2^16), by Henri & Renaud Lifchitz.
- "Generalized Fermat Primes".
*jeppesn.dk*. Retrieved 7 April 2018. - "Generalized Fermat primes for bases up to 1030".
*noprimeleftbehind.net*. Retrieved 7 April 2018. - "Generalized Fermat primes in odd bases".
*fermatquotient.com*. Retrieved 7 April 2018. - Caldwell, Chris K. "The Top Twenty: Generalized Fermat".
*The Prime Pages*. Retrieved 11 July 2019. - Caldwell, Chris K. "Database Search Output".
*The Prime Pages*. Retrieved 11 July 2019. - 1059094
^{1048576}+ 1 - 919444
^{1048576}+ 1 - 2877652
^{524288}+ 1 - 2788032
^{524288}+ 1 - 2733014
^{524288}+ 1

## References

- Golomb, S. W. (January 1, 1963), "On the sum of the reciprocals of the Fermat numbers and related irrationalities",
*Canadian Journal of Mathematics*,**15**: 475–478, doi:10.4153/CJM-1963-051-0 - Grytczuk, A.; Luca, F. & Wójtowicz, M. (2001), "Another note on the greatest prime factors of Fermat numbers",
*Southeast Asian Bulletin of Mathematics*,**25**(1): 111–115, doi:10.1007/s10012-001-0111-4 - Guy, Richard K. (2004),
*Unsolved Problems in Number Theory*, Problem Books in Mathematics,**1**(3rd ed.), New York: Springer Verlag, pp. A3, A12, B21, ISBN 978-0-387-20860-2 - Křížek, Michal; Luca, Florian & Somer, Lawrence (2001),
*17 Lectures on Fermat Numbers: From Number Theory to Geometry*, CMS books in mathematics,**10**, New York: Springer, ISBN 978-0-387-95332-8 - This book contains an extensive list of references. - Křížek, Michal; Luca, Florian & Somer, Lawrence (2002), "On the convergence of series of reciprocals of primes related to the Fermat numbers" (PDF),
*Journal of Number Theory*,**97**(1): 95–112, doi:10.1006/jnth.2002.2782 - Luca, Florian (2000), "The anti-social Fermat number",
*American Mathematical Monthly*,**107**(2): 171–173, doi:10.2307/2589441, JSTOR 2589441 - Ribenboim, Paulo (1996),
*The New Book of Prime Number Records*(3rd ed.), New York: Springer, ISBN 978-0-387-94457-9 - Robinson, Raphael M. (1954), "Mersenne and Fermat Numbers",
*Proceedings of the American Mathematical Society*,**5**(5): 842–846, doi:10.2307/2031878, JSTOR 2031878 - Yabuta, M. (2001), "A simple proof of Carmichael's theorem on primitive divisors" (PDF),
*Fibonacci Quarterly*,**39**: 439–443

## External links

*Fermat prime*at the*Encyclopædia Britannica*- Chris Caldwell, The Prime Glossary: Fermat number at The Prime Pages.
- Luigi Morelli, History of Fermat Numbers
- John Cosgrave, Unification of Mersenne and Fermat Numbers
- Wilfrid Keller, Prime Factors of Fermat Numbers
- Weisstein, Eric W. "Fermat Number".
*MathWorld*. - Weisstein, Eric W. "Fermat Prime".
*MathWorld*. - Weisstein, Eric W. "Fermat Pseudoprime".
*MathWorld*. - Weisstein, Eric W. "Generalized Fermat Number".
*MathWorld*. - Yves Gallot, Generalized Fermat Prime Search
- Mark S. Manasse, Complete factorization of the ninth Fermat number (original announcement)
- Peyton Hayslette, Largest Known Generalized Fermat Prime Announcement