# Sophie Germain 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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## Contents

## Definition

A **Sophie Germain prime** is a prime number such that is also prime. The corresponding prime is termed a safe prime.

## Occurrence

### Initial values

The first few Sophie Germain primes are:

2, 3, 5, 11, 23, 29, 41, 53, 83, 89, [SHOW MORE]View list on OEISThe first few primes that are *not* Sophie Germain primes are: 7, 13, 17, 19, 31.

### Density in primes

Cutoff | Number of primes | Number of Sophie Germain primes | Proportion of primes that are Sophie Germain primes | Number of Sophie Germain primes divided by |
---|---|---|---|---|

10 | 4 | 3 | ||

100 | 25 | 10 | ||

1000 | 168 | 37 |

### Infinitude conjecture

`Further information: Infinitude conjecture for Sophie Germain primes`

It is conjectured that there are infinitely many Sophie Germain primes (or equivalently, that there are infinitely many safe primes). It is also conjectured that the number of Sophie Germain primes less than or equal to is , in other words, there exist constants such that the following holds for all sufficiently large :

.

where denotes the number of Sophie Germain primes less than or equal to .

Other related conjectures to the existence of Sophie Germain primes are:

- Conjecture on existence of Cunningham chains of the first kind of arbitrary length
- Conjecture on existence of bitwin chains of arbitrary length

### Infinitude of complement

Most primes are *not* Sophie Germain primes. It is rather easy to see that the number of primes that are not Sophie Germain primes is infinite: for instance, there are infinitely many primes that are modulo , and none of them can be a Sophie Germain prime.

## Facts

While there are not too many interesting facts about the structure of Sophie Germain primes *per se*, the structure of the corresponding safe primes is very interesting:

- Quadratic nonresidue that is not minus one is primitive root for safe prime
- Safe prime has plus or minus two as primitive root

## Testing

The ID of the sequence in the Online Encyclopedia of Integer Sequences is A005384