# Fermat number

Short description: Positive integer of the form (2^(2^n))+1
Named after Pierre de Fermat 5 5 Fermat numbers 3, 5, 17, 257, 65537 65537 A019434

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

$\displaystyle{ F_{n} = 2^{2^n} + 1, }$

where n is a non-negative integer. The first few Fermat numbers are:

3, 5, 17, 257, 65537, 4294967297, 18446744073709551617, ... (sequence A000215 in the OEIS).

If 2k + 1 is prime and k > 0, then k itself must be a power of 2, so 2k + 1 is a Fermat number; such primes are called Fermat primes. (As of 2023), the only known Fermat primes are F0 = 3, F1 = 5, F2 = 17, F3 = 257, and F4 = 65537 (sequence A019434 in the OEIS); heuristics suggest that there are no more.

## Basic properties

The Fermat numbers satisfy the following recurrence relations:

$\displaystyle{ F_{n} = (F_{n-1}-1)^{2}+1 }$
$\displaystyle{ F_{n} = F_{0} \cdots F_{n-1} + 2 }$

for n ≥ 1,

$\displaystyle{ F_{n} = F_{n-1} + 2^{2^{n-1}}F_{0} \cdots F_{n-2} }$
$\displaystyle{ F_{n} = F_{n-1}^2 - 2(F_{n-2}-1)^2 }$

for n ≥ 2. Each of these relations can be proved by mathematical induction. From the second 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 Fi and Fj have a common factor a > 1. Then a divides both

$\displaystyle{ F_{0} \cdots F_{j-1} }$

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.

### Further properties

• No Fermat prime can be expressed as the difference of two pth powers, where p is an odd prime.
• With the exception of F0 and F1, 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

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. Fermat's conjecture was refuted by Leonhard Euler in 1732 when he showed that

$\displaystyle{ F_{5} = 2^{2^5} + 1 = 2^{32} + 1 = 4294967297 = 641 \times 6700417. }$

Euler proved that every factor of Fn must have the form k2n+1 + 1 (later improved to k2n+2 + 1 by Lucas) for n ≥ 2.

That 641 is a factor of F5 can be deduced from the equalities 641 = 27 × 5 + 1 and 641 = 24 + 54. It follows from the first equality that 27 × 5 ≡ −1 (mod 641) and therefore (raising to the fourth power) that 228 × 54 ≡ 1 (mod 641). On the other hand, the second equality implies that 54 ≡ −24 (mod 641). These congruences imply that 232 ≡ −1 (mod 641).

Fermat was probably aware of the form of the factors later proved by Euler, so it seems curious that 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 Fn with n > 4, but little is known about Fermat numbers for large n.[2] In fact, each of the following is an open problem:

• Is Fn 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 Fn is composite for 5 ≤ n ≤ 32, although of these, complete factorizations of Fn 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 F18233954, and its prime factor 7 × 218233956 + 1 was discovered in October 2020.

### Heuristic arguments

Heuristics suggest that F4 is the last Fermat prime.

The prime number theorem implies that a random integer in a suitable interval around N is prime with probability 1/ln N. If one uses the heuristic that a Fermat number is prime with the same probability as a random integer of its size, and that F5, ..., F32 are composite, then the expected number of Fermat primes beyond F4 (or equivalently, beyond F32) should be

$\displaystyle{ \sum_{n \ge 33} \frac{1}{\ln F_{n}} \lt \frac{1}{\ln 2} \sum_{n \ge 33} \frac{1}{\log_2(2^{2^n})} = \frac{1}{\ln 2} 2^{-32} \lt 3.36 \times 10^{-10}. }$

One may interpret this number as an upper bound for the probability that a Fermat prime beyond F4 exists.

This argument is not a rigorous proof. For one thing, it assumes that Fermat numbers behave "randomly", but the factors of Fermat numbers have special properties. Boklan and Conway published a more precise analysis suggesting that the probability that there is another Fermat prime is less than one in a billion.[5]

Anders Bjorn and Hans Riesel estimated the number of square factors of Fermat numbers from F5 onward as

$\displaystyle{ \sum_{n \ge 5} \sum_{k \ge 1} \frac{1}{k (k 2^n + 1) \ln(k 2^n)} \lt \frac{\pi^2}{6 \ln 2} \sum_{n \ge 5} \frac{1}{n 2^n} \approx 0.02576; }$

in other words, there are unlikely to be any non-squarefree Fermat numbers, and in general square factors of $\displaystyle{ a^{2^n} + b^{2^n} }$ are very rare for large n.[6]

### Equivalent conditions

Let $\displaystyle{ F_n=2^{2^n}+1 }$ be the nth Fermat number. Pépin's test states that for n > 0,

$\displaystyle{ F_n }$ is prime if and only if $\displaystyle{ 3^{(F_n-1)/2}\equiv-1\pmod{F_n}. }$

The expression $\displaystyle{ 3^{(F_n-1)/2} }$ can be evaluated modulo $\displaystyle{ F_n }$ by repeated squaring. This makes the test a fast polynomial-time algorithm. But Fermat numbers grow so rapidly that only a handful of them can be tested in a reasonable amount of time and space.

There are some tests for numbers of the form k2m + 1, such as factors of Fermat numbers, for primality.

Proth's theorem (1878). Let N = k2m + 1 with odd k < 2m. If there is an integer a such that
$\displaystyle{ a^{(N-1)/2} \equiv -1\pmod{N} }$
then $\displaystyle{ N }$ is prime. Conversely, if the above congruence does not hold, and in addition
$\displaystyle{ \left(\frac{a}{N}\right)=-1 }$ (See Jacobi symbol)
then $\displaystyle{ N }$ 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 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

Because of Fermat numbers' size, 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 found some factors of Fermat numbers. Yves Gallot's proth.exe has been used to find factors of large Fermat numbers. Édouard Lucas, improving Euler's above-mentioned result, proved in 1878 that every factor of the Fermat number $\displaystyle{ F_n }$, with n at least 2, is of the form $\displaystyle{ k\times2^{n+2}+1 }$ (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:

 F0 = 21 + 1 = 3 is prime F1 = 22 + 1 = 5 is prime F2 = 24 + 1 = 17 is prime F3 = 28 + 1 = 257 is prime F4 = 216 + 1 = 65,537 is the largest known Fermat prime F5 = 232 + 1 = 4,294,967,297 = 641 × 6,700,417 (fully factored 1732[7]) F6 = 264 + 1 = 18,446,744,073,709,551,617 (20 digits) = 274,177 × 67,280,421,310,721 (14 digits) (fully factored 1855) F7 = 2128 + 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) F8 = 2256 + 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) F9 = 2512 + 1 = 13,407,807,929,942,597,099,574,024,998,205,846,127,479,365,820,592,393,377,723,561,443,721,764,030,073,546,976,801,874,298,166,903,427,690,031,858,186,486,050,853,753,882,811,946,569,946,433,649,006,084,097 (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) F10 = 21024 + 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) F11 = 22048 + 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 April 2023), only F0 to F11 have been completely factored.[4] The distributed computing project Fermat Search is searching for new factors of Fermat numbers.[8] The set of all Fermat factors is A050922 (or, sorted, A023394) in OEIS.

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

Year Finder Fermat number Factor
1732 Euler $\displaystyle{ F_5 }$ $\displaystyle{ 5 \cdot 2^7 + 1 }$
1732 Euler $\displaystyle{ F_5 }$ (fully factored) $\displaystyle{ 52347 \cdot 2^7 + 1 }$
1855 Clausen $\displaystyle{ F_6 }$ $\displaystyle{ 1071 \cdot 2^8 + 1 }$
1855 Clausen $\displaystyle{ F_6 }$ (fully factored) $\displaystyle{ 262814145745 \cdot 2^8 + 1 }$
1877 Pervushin $\displaystyle{ F_{12} }$ $\displaystyle{ 7 \cdot 2^{14} + 1 }$
1878 Pervushin $\displaystyle{ F_{23} }$ $\displaystyle{ 5 \cdot 2^{25} + 1 }$
1886 Seelhoff $\displaystyle{ F_{36} }$ $\displaystyle{ 5 \cdot 2^{39} + 1 }$
1899 Cunningham $\displaystyle{ F_{11} }$ $\displaystyle{ 39 \cdot 2^{13} + 1 }$
1899 Cunningham $\displaystyle{ F_{11} }$ $\displaystyle{ 119 \cdot 2^{13} + 1 }$
1903 Western $\displaystyle{ F_9 }$ $\displaystyle{ 37 \cdot 2^{16} + 1 }$
1903 Western $\displaystyle{ F_{12} }$ $\displaystyle{ 397 \cdot 2^{16} + 1 }$
1903 Western $\displaystyle{ F_{12} }$ $\displaystyle{ 973 \cdot 2^{16} + 1 }$
1903 Western $\displaystyle{ F_{18} }$ $\displaystyle{ 13 \cdot 2^{20} + 1 }$
1903 Cullen $\displaystyle{ F_{38} }$ $\displaystyle{ 3 \cdot 2^{41} + 1 }$
1906 Morehead $\displaystyle{ F_{73} }$ $\displaystyle{ 5 \cdot 2^{75} + 1 }$
1925 Kraitchik $\displaystyle{ F_{15} }$ $\displaystyle{ 579 \cdot 2^{21} + 1 }$

(As of July 2023), 368 prime factors of Fermat numbers are known, and 324 Fermat numbers are known to be composite.[4] Several new Fermat factors are found each year.[9]

## Pseudoprimes and Fermat numbers

Like composite numbers of the form 2p − 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.,

$\displaystyle{ 2^{F_n-1} \equiv 1 \pmod{F_n} }$

for all Fermat numbers.

In 1904, Cipolla showed that the product of at least two distinct prime or composite Fermat numbers $\displaystyle{ F_{a} F_{b} \dots F_{s}, }$ $\displaystyle{ a \gt b \gt \dots \gt s \gt 1 }$ will be a Fermat pseudoprime to base 2 if and only if $\displaystyle{ 2^s \gt a }$.[10]

## Other theorems about Fermat numbers

Lemma. — If n is a positive integer,

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

Theorem —  If $\displaystyle{ 2^k+1 }$ is an odd prime, then $\displaystyle{ k }$ is a power of 2.

Theorem —  A Fermat prime cannot be a Wieferich prime.

Theorem  —  Any prime divisor p of $\displaystyle{ F_n = 2^{2^n}+1 }$ is of the form $\displaystyle{ k2^{n+2}+1 }$ whenever n > 1.

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)

If nn + 1 is prime, there exists an integer m such that n = 22m. The equation nn + 1 = F(2m+m) holds in that case.[11][12]

Let the largest prime factor of the Fermat number Fn be P(Fn). Then,

$\displaystyle{ P(F_n) \ge 2^{n+2}(4n+9) + 1. }$ (Grytczuk Luca)

## Relationship to constructible polygons

Number of sides of known constructible polygons having up to 1000 sides (bold) or odd side count (red)

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 that this condition was also necessary,[13] but never published a proof. Pierre Wantzel gave a full proof of necessity 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 either a power of 2 or the product of a power of 2 and distinct Fermat primes: in other words, if and only if n is of the form n = 2k or n = 2kp1p2...ps, where k, s are nonnegative integers and the pi 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.

$\displaystyle{ V_{j+1} = (A \times V_j) \bmod P }$ (see linear congruential generator, RANDU)

This is useful in computer science, since most data structures have members with 2X 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 that 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.

## Generalized Fermat numbers

Numbers of the form $\displaystyle{ a^{2^{ \overset{n} {}}} \!\!+ b^{2^{ \overset{n} {}}} }$ 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 = $\displaystyle{ 2^{2^{0}} \!+ 1 }$ is not a counterexample.)

An example of a probable prime of this form is 1215131072 + 242131072 (found by Kellen Shenton).[14]

By analogy with the ordinary Fermat numbers, it is common to write generalized Fermat numbers of the form $\displaystyle{ a^{2^{ \overset{n} {}}} \!\!+ 1 }$ as Fn(a). In this notation, for instance, the number 100,000,001 would be written as F3(10). In the following we shall restrict ourselves to primes of this form, $\displaystyle{ a^{2^{ \overset{n} {}}} \!\!+ 1 }$, 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 Fn(a).

### Generalized Fermat primes of the form Fn(a)

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 $\displaystyle{ F_n(a) }$ with $\displaystyle{ n\gt 4 }$ is $\displaystyle{ F_5(30) }$, or 3032 + 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 $\displaystyle{ \frac{a^{2^n} \!+ 1}{2} }$, and it is also to be expected that there will be only finitely many half generalized Fermat primes for each odd base.

In this list, the generalized Fermat numbers ($\displaystyle{ F_n(a) }$) to an even a are $\displaystyle{ a^{2^n} \!+ 1 }$, for odd a, they are $\displaystyle{ \frac{a^{2^n} \!\!+ 1}{2} }$. 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.

See[15][16] for even bases up to 1000, and[17] for odd bases. For the smallest number $\displaystyle{ n }$ such that $\displaystyle{ F_n(a) }$ is prime, see .

$\displaystyle{ a }$ numbers $\displaystyle{ n }$
such that
$\displaystyle{ F_n(a) }$ is prime
$\displaystyle{ a }$ numbers $\displaystyle{ n }$
such that
$\displaystyle{ F_n(a) }$ is prime
$\displaystyle{ a }$ numbers $\displaystyle{ n }$
such that
$\displaystyle{ F_n(a) }$ is prime
$\displaystyle{ a }$ numbers $\displaystyle{ n }$
such that
$\displaystyle{ F_n(a) }$ 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, ...

For the smallest even base a such that $\displaystyle{ F_n(a) }$ is prime, see .

$\displaystyle{ n }$ bases a such that $\displaystyle{ F_n(a) }$ 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, 2733014, 2788032, 2877652, 2985036, 3214654, 3638450, 4896418, 5897794, ... A243959
20 919444, 1059094, 1951734, 1963736, ... A321323

The smallest bases b=b(n) such that b2n + 1 (for given n= 0,1,2, ...) 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)

Conversely, the smallest k=k(n) such that (2n)k + 1 (for given n) 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 and )

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

### Generalized Fermat primes of the form Fn(a, b)

It is also possible to construct generalized Fermat primes of the form $\displaystyle{ a^{2^n} + b^{2^n} }$. As in the case where b=1, numbers of this form will always be divisible by 2 if a+b is even, but it is still possible to define generalized half-Fermat primes of this type. For the smallest prime of the form $\displaystyle{ F_n(a,b) }$ (for odd $\displaystyle{ a+b }$), see also .

$\displaystyle{ a }$ $\displaystyle{ b }$ numbers $\displaystyle{ n }$ such that
$\displaystyle{ F_n(a,b) = \frac{a^{2^n}+b^{2^n}}{\gcd(a+b, 2)} }$
is prime[18][6]
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, ... (equivalent to $\displaystyle{ F_n(2, 1) }$)
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, ... (equivalent to $\displaystyle{ F_n(3, 1) }$)
9 2 0, 2, ...
9 4 0, 1, ... (equivalent to $\displaystyle{ F_n(3, 2) }$)
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, ...

### Largest known generalized Fermat primes

The following is a list of the five largest known generalized Fermat primes.[19] The whole top-5 is discovered by participants in the PrimeGrid project.

Rank Prime number Generalized Fermat notation Number of digits Discovery date ref.
1 19637361048576 + 1 F20(1963736) 6,598,776 Sep 2022 [20]
2 19517341048576 + 1 F20(1951734) 6,595,985 Aug 2022 [21]
3 10590941048576 + 1 F20(1059094) 6,317,602 Nov 2018 [22]
4 9194441048576 + 1 F20(919444) 6,253,210 Sep 2017 [23]
5 81 × 220498148 + 1 F2(3 × 25124537) 6,170,560 Jun 2023 [24]

On the Prime Pages one can find the current top 100 generalized Fermat primes.

## Notes

1. Křížek, Luca & Somer 2001, p. 38, Remark 4.15
2. Chris Caldwell, "Prime Links++: special forms" at The Prime Pages.
3. Ribenboim 1996, p. 88.
4. Keller, Wilfrid (January 18, 2021), "Prime Factors of Fermat Numbers", ProthSearch.com, retrieved January 19, 2021
5. Boklan, Kent D.; Conway, John H. (2017). "Expect at most one billionth of a new Fermat Prime!". The Mathematical Intelligencer 39 (1): 3–5. doi:10.1007/s00283-016-9644-3.
6. Björn, Anders; Riesel, Hans (1998). "Factors of generalized Fermat numbers" (in en). Mathematics of Computation 67 (221): 441–446. doi:10.1090/S0025-5718-98-00891-6. ISSN 0025-5718.
7. Sandifer, Ed. "How Euler Did it". Mathematical Association of America.
8. 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.
9. Jeppe Stig Nielsen, "S(n) = n^n + 1".
10. Gauss, Carl Friedrich (1966). Disquisitiones arithmeticae. New Haven and London: Yale University Press. pp. 458–460. Retrieved 25 January 2023.
11. PRP Top Records, search for x^131072+y^131072, by Henri & Renaud Lifchitz.
12. Caldwell, Chris K.. "The Top Twenty: Generalized Fermat". The Prime Pages.
13. 19637361048576 + 1
14. 19517341048576 + 1
15. 10590941048576 + 1
16. 9194441048576 + 1
17. 81*220498148 + 1