# Fermat prime

This article defines a property that can be evaluated for a prime number. In other words, every prime number either satisfies this property or does not satisfy this property.
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## Definition

A Fermat prime is a Fermat number that is also a prime number. In other words, it is a prime number of the form , where  is a nonnegative integer.

It turns out that if  is prime for  a natural number, then  for some nonnegative integer . Thus, a Fermat prime can also be defined as a prime of the form  for some natural number .

## Occurrence

### Finitude conjecture

Further information: Finitude conjecture for Fermat primes

The only known Fermat primes are the Fermat primes for , namely, the primes . For all , either the Fermat prime  is known to be composite or its primality is open. The prime number theorem suggests that there are likely to be only finitely many Fermat primes, and it is conjectured that there are no Fermat primes other than the five known ones.

The ID of the sequence in the Online Encyclopedia of Integer Sequences is A019434

## Testing

### Test for this property

The test used to determine whether a natural number has this property is: Pepin's test

Pepin's test is a deterministic primality test for Fermat numbers. It relies on Proth's theorem, and the observation that  is a quadratic nonresidue for any Fermat prime greater than  itself.