Fermat number

In mathematics, a Fermat number, named after Pierre de Fermat (1607–1665), the first known to have studied them, is a positive integer of the form:$F_n=2^{2^n}+1,$ where n is a non-negative integer. The first few Fermat numbers are: 3, 5, 17, 257, 65537, 4294967297, 18446744073709551617, 340282366920938463463374607431768211457, ... Template:OEIS.

If 2^k + 1 is prime and Template:Nowrap, then k itself must be a power of 2, so Template:Nowrap is a Fermat number; such primes are called Fermat primes. Template:As of, the only known Fermat primes are Template:Nowrap, Template:Nowrap, Template:Nowrap, Template:Nowrap, and Template:Nowrap Template:OEIS.

Basic properties

The Fermat numbers satisfy the following recurrence relations:

$F_n=(F_{n-1}-1{)}^2+1$

$F_n=F_0\cdots F_{n-1}+2$

for n ≥ 1,

$F_n=F_{n-1}+2^{2^{n-1}}{F}_0\cdots F_{n-2}$

$F_n=F_{n-1}^2-2(F_{n-2}-1{)}^2$

for Template:Nowrap. 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 Template:Nowrap and F_i and F_j have a common factor Template:Nowrap. Then a divides both

$F_0\cdots F_{j-1}$

and F_j; hence a divides their difference, 2. Since Template:Nowrap, this forces Template:Nowrap. 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 Template:Mset 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 decimal digit of a Fermat number is 7.
• The sum of the reciprocals of all the Fermat numbers Template: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

$F_5=2^{2^5}+1=2^{32}+1=4294967297=641\times 6700417.$

Euler proved that every factor of F_n must have the form Template:Nowrap (later improved to Template:Nowrap by Lucas) for Template:Nowrap.

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. One common explanation is that Fermat made a computational mistake.

There are no other known Fermat primes F_n with Template:Nowrap, but little is known about Fermat numbers for large n. In fact, each of the following is an open problem:

• Is F_n composite for all Template:Nowrap?
• Are there infinitely many Fermat primes? (Eisenstein 1844)
• Are there infinitely many composite Fermat numbers?
• Does a Fermat number exist that is not square-free?

Template:As of, it is known that F_n is composite for Template:Nowrap, although of these, complete factorizations of F_n are known only for Template:Nowrap, and there are no known prime factors for Template:Nowrap and Template:Nowrap. The largest Fermat number known to be composite is F_18233954, and its prime factor Template:Nowrap 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 1Template:Space/Template:Spaceln 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

$\sum _{n\ge 33}\frac{1}{\ln F_n}<\frac{1}{\ln 2}\sum _{n\ge 33}\frac{1}{{\log }_2(2^{2^n})}=\frac{1}{\ln 2}{2}^{-32}<3.36\times 10^{-10}.$

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.

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

$\sum _{n\ge 5}\sum _{k\ge 1}\frac{1}{k(k{2}^n+1)\ln (k{2}^n)}<\frac{{\pi }^2}{6\ln 2}\sum _{n\ge 5}\frac{1}{n{2}^n}\approx 0.02576;$

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

Equivalent conditions

Let $F_n=2^{2^n}+1$ be the nth Fermat number. Pépin's test states that for Template:Nowrap,

$F_n$ is prime if and only if $3^{(F_n-1)/2}\equiv -1(\mathrm{mod}{F}_n).$

The expression $3^{(F_n-1)/2}$ can be evaluated modulo $F_n$ 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 Template:Nowrap, such as factors of Fermat numbers, for primality.

Proth's theorem (1878). Let Template:Nowrap with odd Template:Nowrap. If there is an integer a such that

$a^{(N-1)/2}\equiv -1(\mathrm{mod}N)$

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 Template:Nowrap, then the above Jacobi symbol is always equal to −1 for Template:Nowrap, 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 Template:Nowrap 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 $F_n$, with n at least 2, is of the form $k\times 2^{n+2}+1$ (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 12 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)
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	=	13,407,807,929,942,597,099,574,024,998,205,846,127,479,365,820,592,393,377,723,561,443,721,764,0 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)
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)

Template:As of, only F₀ to F₁₁ have been completely factored. The distributed computing project Fermat Search is searching for new factors of Fermat numbers. 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	$F_5$	$5\cdot 2^7+1$
1732	Euler	$F_5$ (fully factored)	$52347\cdot 2^7+1$
1855	Clausen	$F_6$	$1071\cdot 2^8+1$
1855	Clausen	$F_6$ (fully factored)	$262814145745\cdot 2^8+1$
1877	Pervushin	$F_{12}$	$7\cdot 2^{14}+1$
1878	Pervushin	$F_{23}$	$5\cdot 2^{25}+1$
1886	Seelhoff	$F_{36}$	$5\cdot 2^{39}+1$
1899	Cunningham	$F_{11}$	$39\cdot 2^{13}+1$
1899	Cunningham	$F_{11}$	$119\cdot 2^{13}+1$
1903	Western	$F_9$	$37\cdot 2^{16}+1$
1903	Western	$F_{12}$	$397\cdot 2^{16}+1$
1903	Western	$F_{12}$	$973\cdot 2^{16}+1$
1903	Western	$F_{18}$	$13\cdot 2^{20}+1$
1903	Cullen	$F_{38}$	$3\cdot 2^{41}+1$
1906	Morehead	$F_{73}$	$5\cdot 2^{75}+1$
1925	Kraitchik	$F_{15}$	$579\cdot 2^{21}+1$

Template:As of, 371 prime factors of Fermat numbers are known, and 324 Fermat numbers are known to be composite. Several new Fermat factors are found each year.

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.,

$2^{F_n-1}\equiv 1(\mathrm{mod}{F}_n)$

for all Fermat numbers.

In 1904, Cipolla showed that the product of at least two distinct prime or composite Fermat numbers $F_a{F}_b\dots F_s,$ $a>b>\dots >s>1$ will be a Fermat pseudoprime to base 2 if and only if $2^s>a$.

Other theorems about Fermat numbers

A Fermat number cannot be a perfect number or part of a pair of amicable numbers. Template:Harv

The series of reciprocals of all prime divisors of Fermat numbers is convergent. Template:Harv

If Template:Nowrap is prime and $n\ge 2$, there exists an integer m such that Template:Nowrap. The equation Template:Nowrap holds in that case.

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

$P(F_n)\ge 2^{n+2}(4n+9)+1.$ Template:Harv

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, 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 Template:Nowrap or Template:Nowrap, where k, s are nonnegative integers 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 Template:Nowrap, 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.

$V_{j+1}=(A\times V_j)modP$ (see linear congruential generator)

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 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 Template:Nowrap repetitions, the sequence repeats. A poorly chosen multiplier can result in the sequence repeating sooner than Template:Nowrap.

Generalized Fermat numbers

Numbers of the form $\frac{a^{2^n}+b^{2^n}}{gcd(a+b,2)}$ with a, b any coprime integers, Template:Nowrap, 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 Template:Nowrap, so Template:Nowrap is not a counterexample.)

An example of a probable prime of this form is 200²⁶²¹⁴⁴ + 119²⁶²¹⁴⁴ (found by Kellen Shenton).

By analogy with the ordinary Fermat numbers, it is common to write generalized Fermat numbers of the form $a^{2^{\stackrel{n}{}}}+1$ as F_n(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, $a^{2^{\stackrel{n}{}}}+1$, such primes are called "Fermat primes base a". Of course, these primes exist only if a is even.

If we require Template:Nowrap, then Landau's fourth problem asks if there are infinitely many generalized Fermat primes F_n(a).

Generalized Fermat primes of the form F_n(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 Template:Mvar, because if Template:Mvar is odd then every generalized Fermat number will be divisible by 2. The smallest prime number $F_n(a)$ with $n>4$ is $F_5(30)$, 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 $\frac{a^{2^n}+1}{2}$, 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 ($F_n(a)$) to an even Template:Mvar are $a^{2^n}+1$, for odd Template:Mvar, they are $\frac{a^{2^n}+1}{2}$. If Template:Mvar is a perfect power with an odd exponent Template:OEIS, then all generalized Fermat number can be algebraic factored, so they cannot be prime.

See for even bases up to 1000, and for odd bases. For the smallest number $n$ such that $F_n(a)$ is prime, see Template:Oeis.

$a$	numbers $n$ such that $F_n(a)$ is prime	$a$	numbers $n$ such that $F_n(a)$ is prime	$a$	numbers $n$ such that $F_n(a)$ is prime	$a$	numbers $n$ such that $F_n(a)$ 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	Template:CNone	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	Template:CNone	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	Template:CNone	48	2, ...	64	Template:CNone
17	2, ...	33	0, 3, ...	49	1, ...	65	1, 2, 5, ...

For the smallest even base Template:Mvar such that $F_n(a)$ is prime, see Template:Oeis.

The generalized Fermat prime F₁₄(71) is the largest known generalized Fermat prime in bases b ≤ 1000, it is proven prime by elliptic curve primality proving.

$n$	bases Template:Mvar such that $F_n(a)$ is prime (only consider even Template:Mvar)	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, ...	Template:OEIS link
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, ...	Template:OEIS link
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, ...	Template:OEIS link
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, ...	Template:OEIS link
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, ...	Template:OEIS link
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, ...	Template:OEIS link
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, ...	Template:OEIS link
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, ...	Template:OEIS link
8	278, 614, 892, 898, 1348, 1494, 1574, 1938, 2116, 2122, 2278, 2762, 3434, 4094, 4204, 4728, 5712, 5744, 6066, 6508, 6930, 7022, 7332, ...	Template:OEIS link
9	46, 1036, 1318, 1342, 2472, 2926, 3154, 3878, 4386, 4464, 4474, 4482, 4616, 4688, 5374, 5698, 5716, 5770, 6268, 6386, 6682, 7388, 7992, ...	Template:OEIS link
10	824, 1476, 1632, 2462, 2484, 2520, 3064, 3402, 3820, 4026, 6640, 7026, 7158, 9070, 12202, 12548, 12994, 13042, 15358, 17646, 17670, ...	Template:OEIS link
11	150, 2558, 4650, 4772, 11272, 13236, 15048, 23302, 26946, 29504, 31614, 33308, 35054, 36702, 37062, 39020, 39056, 43738, 44174, 45654, ...	Template:OEIS link
12	1534, 7316, 17582, 18224, 28234, 34954, 41336, 48824, 51558, 51914, 57394, 61686, 62060, 89762, 96632, 98242, 100540, 101578, 109696, ...	Template:OEIS link
13	30406, 71852, 85654, 111850, 126308, 134492, 144642, 147942, 150152, 165894, 176206, 180924, 201170, 212724, 222764, 225174, 241600, ...	Template:OEIS link
14	67234, 101830, 114024, 133858, 162192, 165306, 210714, 216968, 229310, 232798, 422666, 426690, 449732, 462470, 468144, 498904, 506664, ...	Template:OEIS link
15	70906, 167176, 204462, 249830, 321164, 330716, 332554, 429370, 499310, 524552, 553602, 743788, 825324, 831648, 855124, 999236, 1041870, 1074542, 1096382, 1113768, 1161054, 1167528, 1169486, 1171824, 1210354, 1217284, 1277444, 1519380, 1755378, 1909372, 1922592, 1986700, ...	Template:OEIS link
16	48594, 108368, 141146, 189590, 255694, 291726, 292550, 357868, 440846, 544118, 549868, 671600, 843832, 857678, 1024390, 1057476, 1087540, 1266062, 1361846, 1374038, 1478036, 1483076, 1540550, 1828502, 1874512, 1927034, 1966374, ...	Template:OEIS link
17	62722, 130816, 228188, 386892, 572186, 689186, 909548, 1063730, 1176694, 1361244, 1372930, 1560730, 1660830, 1717162, 1722230, 1766192, 1955556, 2194180, 2280466, 2639850, 3450080, 3615210, 3814944, 4085818, 4329134, 4893072, 4974408, ...	Template:OEIS link
18	24518, 40734, 145310, 361658, 525094, 676754, 773620, 1415198, 1488256, 1615588, 1828858, 2042774, 2514168, 2611294, 2676404, 3060772, 3547726, 3596074, 3673932, 3853792, 3933508, 4246258, 4489246, ...	Template:OEIS link
19	75898, 341112, 356926, 475856, 1880370, 2061748, 2312092, 2733014, 2788032, 2877652, 2985036, 3214654, 3638450, 4896418, 5897794, 6339004, 8630170, 9332124, 10913140, 11937916, 12693488, 12900356, ...	Template:OEIS link
20	919444, 1059094, 1951734, 1963736, 3843236, ...	Template:OEIS link

The smallest even base b such that F_n(b) = 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, ... Template:OEIS

The smallest odd base b such that F_n(b) = (b²ⁿ + 1)/2 (for given n = 0, 1, 2, ...) is prime (or probable prime) are

3, 3, 3, 9, 3, 3, 3, 113, 331, 513, 827, 799, 3291, 5041, 71, 220221, 23891, 11559, 187503, 35963, ... Template:OEIS

Conversely, the smallest k 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) Template:OEIS (also see Template:Oeis and Template:Oeis)

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

Generalized Fermat primes of the form F_n(a, b)

It is also possible to construct generalized Fermat primes of the form $a^{2^n}+b^{2^n}$. 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 $F_n(a,b)$ (for odd $a+b$), see also Template:Oeis.

$a$	$b$	numbers $n$ such that $F_n(a,b)=\frac{a^{2^n}+b^{2^n}}{\gcd (a+b,2)}$ 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, ... (equivalent to $F_n(2,1)$)
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	Template:CNone
8	3	0, 1, 2, ...
8	5	0, 1, 2,
8	7	1, 4, ...
9	1	0, 1, 3, 4, 5, ... (equivalent to $F_n(3,1)$)
9	2	0, 2, ...
9	4	0, 1, ... (equivalent to $F_n(3,2)$)
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 ten largest known generalized Fermat primes. The whole top-10 is discovered by participants in the PrimeGrid project.

Rank	Prime number	Generalized Fermat notation	Number of digits	Discovery date	ref.
1	4×511786358 + 1	F1(2×55893179)	8,238,312	Oct 2024
2	38432361048576 + 1	F20(3843236)	6,904,556	Dec 2024
3	19637361048576 + 1	F20(1963736)	6,598,776	Sep 2022
4	19517341048576 + 1	F20(1951734)	6,595,985	Aug 2022
5	10590941048576 + 1	F20(1059094)	6,317,602	Nov 2018
6	9194441048576 + 1	F20(919444)	6,253,210	Sep 2017
7	81×220498148 + 1	F2(3×25124537)	6,170,560	Jun 2023
8	4×58431178 + 1	F1(2×54215589)	5,893,142	Jan 2024
9	4×311279466 + 1	F1(2×35639733)	5,381,674	Sep 2024
10	25×213719266 + 1	F1(5×26859633)	4,129,912	Sep 2022

On the Prime Pages one can find the current top 20 generalized Fermat primes and 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

References

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External links

• 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
• Template:MathWorld
• Template:MathWorld
• Template: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