# Definition:Fermat Number

## Contents

## Definition

A **Fermat number** is a natural number of the form $2^{\paren {2^n} } + 1$, where $n = 0, 1, 2, \ldots$.

The number $2^{\paren {2^n} } + 1$ is, in this context, often denoted $F_n$.

### Sequence

The sequence of **Fermat numbers** begins:

- $3, 5, 17, 257, 65 \, 537, 4 \, 294 \, 967 \, 297, 18 \, 446 \, 744 \, 073 \, 709 \, 551 \, 617, \ldots$

This sequence is A000215 in the On-Line Encyclopedia of Integer Sequences (N. J. A. Sloane (Ed.), 2008).

## Naming Conventions

The **Fermat number** $F_0$ is often referred to as the ** $1$st Fermat number**, so (confusingly) this convention dictates that $F_n$ is the **$n + 1$th Fermat number**.

However, another convention is that $F_0$ can be referred to as the **zeroth Fermat number**, thus bringing the appellation in line such that $F_n$ is the **$n$th Fermat number**.

Both conventions are in place, sometimes in the same work.

For example, David Wells, in his *Curious and Interesting Numbers, 2nd ed.* of $1997$, refers to $5 = F_1$ in Section $5$ as the **$2$nd Fermat number**.

However, in Section $257$ he defines $F_3 = 2^{2^3} + 1 = 257$ as the **$3$rd Fermat number**.

Similarly, in Section $65,537$ he defines $F_4 = 2^{2^4} + 1 = 65 \, 537$ as the **$4$th Fermat number**, and so on.

Both of these naming conventions is more or less clumsy.

$\mathsf{Pr} \infty \mathsf{fWiki}$ takes the position that the cat has to jump one way or the other, and so uses the second of these conventions:

- $F_n$ is the
**$n$th Fermat number**.

## Also see

- Results about
**Fermat Numbers**can be found here.

## Source of Name

This entry was named for Pierre de Fermat.

## Historical Note

In $1640$, Pierre de Fermat wrote to Bernard Frénicle de Bessy that $2^n + 1$ is composite if $n$ is divisible by an odd prime.

He also observed that the first $5$ numbers of the form $2^{2^n} + 1$ are all prime.

This led him to propose the Fermat Prime Conjecture: that all numbers of this form are prime.

On being unable to prove it, he sent the problem to Blaise Pascal, with the note:

*I wouldn't ask you to work at it if I had been successful.*

Pascal unfortunately did not take up the challenge.

The Fermat Prime Conjecture was proved false by Leonhard Paul Euler, who discovered the prime decomposition of the $6$th Fermat number $F_5$.

In $1877$, Ivan Mikheevich Pervushin proved that $F_{12}$ is divisible by $7 \times 2^{14} + 1 = 114 \, 689$, but was unable to completely factorise it.

In $1878$, he similarly found that $5 \times 2^{25} + 1$ is a divisor of $F_{23}$.

Fortuné Landry factorised $F_6$ in $1880$, in the process setting the still-unbroken record for finding the largest non-Mersenne prime number without the use of a computer.

In $1909$, James C. Morehead and Alfred E. Western reported in *Bulletin of the American Mathematical Society* that they had proved that $F_7$ and $F_8$ are not prime, but without having established what the prime factors are.

Prior to that, several divisors of various Fermat numbers had been identified, including $F_{73}$ by Morehead, who found the divisor $5 \times 2^{75} + 1$ in $1906$.

The prime factors of $F_7$ were finally discovered by Michael A. Morrison and John David Brillhart in $1970$:

- $F_7 = \left({116 \, 503 \, 103 \, 764 \, 643 \times 2^9 + 1}\right) \left({11 \, 141 \, 971 \, 095 \, 088 \, 142 \, 685 \times 2^9 + 1}\right)$

One of the divisors of $F_8$ was found by Richard Peirce Brent and John Michael Pollard in $1981$:

- $1 \, 238 \, 926 \, 361 \, 552 \, 897$

Some divisors of truly colossal Fermat numbers are known.

For example:

- a divisor of $F_{1945}$ is known
- $19 \times 2^{9450} + 1$ is a divisor of $F_{9448}$
- $5 \times 2^{23 \, 473} + 1$ is a divisor of $F_{23 \, 471}$

## Sources

- 1937: Eric Temple Bell:
*Men of Mathematics*... (previous) ... (next): Chapter $\text{IV}$: The Prince of Amateurs - 1982: P.M. Cohn:
*Algebra Volume 1*(2nd ed.) ... (previous) ... (next): $\S 2.4$: The rational numbers and some finite fields: Further Exercises $8$ - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $5$ - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $127$ - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $257$ - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $65,537$ - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $4,294,967,297$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $5$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $127$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $257$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $65,537$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $4,294,967,297$