# Fermats little theorem

**Fermat's little theorem** (not to be confused with Fermat's last theorem) states that if *p* is a prime number, then for any integer *a*,

**Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a^p \equiv a \pmod{p}\,\!}**

This means that if you take some number *a*, multiply it by itself *p* times and subtract *a*, the result is divisible by *p* (see modular arithmetic).

The theorem is often stated in the following equivalent form: if *p* is a prime and *a* is an integer coprime to *p*, then

**Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a^{p-1} \equiv 1 \pmod{p}\,\!}**

Fermat's little theorem is the basis for the Fermat primality test.

## Contents

## Numerical examples

Examples of the theorem include:

- 4
^{3}− 4 = 60 is divisible by 3. - (−3)
^{7}− (−3) = −2184 is divisible by 7. - 2
^{97}− 2 = 158456325028528675187087900670 is divisible by 97.

## History

Pierre de Fermat found the theorem around 1636. It appeared in one of his letters, dated October 18 1640 to his confidant Frenicle as the following: *p* divides **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a^{p-1}-1\,}**
whenever *p* is prime and *a* is coprime to *p*.

Chinese mathematicians independently made the related hypothesis (sometimes called the Chinese Hypothesis) that *p* is a prime if and only if **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 2^p \equiv 2 \pmod{p}\,}**
. It is true that if *p* is prime, then **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 2^p \equiv 2 \pmod{p}\,}**
(this is a special case of Fermat's little theorem). However, the converse (if **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \,2^p \equiv 2 \pmod{p}}**
then *p* is prime), and therefore the hypothesis as a whole, is false (e.g. 341=11×31 is a pseudoprime, see below).

It is widely stated that the Chinese hypothesis was developed about 2000 years before Fermat's work in the 1600's. Despite the fact that the hypothesis is partially incorrect, it is noteworthy that it may have been known to ancient mathematicians. Some, however, claim that the widely propagated belief that the hypothesis was around so early sprouted from a misunderstanding, and that it was actually developed in 1872. For more on this, see (Ribenboim, 1995).

## Proofs

Fermat explained his theorem without a proof. The first one who gave a proof was Gottfried Wilhelm Leibniz in a manuscript without a date, where he wrote also that he knew a proof before 1683.

See Proofs of Fermat's little theorem.

## Generalizations

A slight generalization of the theorem, which immediately follows from it, is as follows: if *p* is prime and *m* and *n* are *positive* integers with **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle m\equiv n\pmod{p-1}\,}**
, then **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a^m\equiv a^n\pmod{p} \quad\forall a\in\mathbb{Z}.}**
In this form, the theorem is used to justify the RSA public key encryption method.

Fermat's little theorem is generalized by Euler's theorem: for any modulus *n* and any integer *a* coprime to *n*, we have

**Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a^{\varphi (n)} \equiv 1 \pmod{n}}**

where φ(*n*) denotes Euler's φ function counting the integers between 1 and *n* that are coprime to *n*. This is indeed a generalization, because if *n* = *p* is a prime number, then φ(*p*) = *p* − 1.

This can be further generalized to Carmichael's theorem.

The theorem has a nice generalization also in finite fields.

## Pseudoprimes

If *a* and *p* are coprime numbers such that **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \,a^{p-1} - 1}**
is divisible by *p*, then *p* need not be prime. If it is not, then *p* is called a pseudoprime to base *a*. F. Sarrus in 1820 found 341 = 11×31 as one of the first pseudoprimes, to base 2.

A number *p* that is a pseudoprime to base *a* for every number *a* coprime to *p* is called a Carmichael number (e.g. 561 is a Carmichael number).

## See also

- Fractions with prime denominators – numbers with behaviour that relates to Fermat's little theorem.
- RSA – How Fermat's little theorem is essential to internet security.

## References

- Ribenboim, P. (1995).
*The New Book of Prime Number Records*(3rd ed.). New York: Springer-Verlag. ISBN 0-387-94457-5. - János Bolyai and the pseudoprimes (in Hungarian)

## External links

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