# 7

## Summary

### Factorization

The number 7 is a prime number.

### Properties and families

Property or family Parameter values First few numbers Proof of satisfaction/membership/containment
prime number fourth prime number 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, [SHOW MORE]View list on OEIS divide and check
Mersenne number  , i.e.,  plug and check
Mersenne prime (both a prime number and a Mersenne number) same as for Mersenne number combine above
safe prime (odd prime such that half of that minus one is also prime) second safe prime 5, 7, 11, 23, 47, 59, 83, 107, 167, [SHOW MORE]View list on OEIS plug and check  is prime.
regular prime third regular prime 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 41, 43, 47, 53, 61, [SHOW MORE]View list on OEIS
twin prime (bigger member of pair) third twin prime, second bigger member twin prime Twin prime 7 - 2 = 5 is prime

## Structure of integers mod 7

### Discrete logarithm

3 is a primitive root mod 7, so we can take that as the base of the discrete logarithm. We thus get an explicit bijection from the additive group of integers mod 6 to the multiplicative group of nonzero congruence classes mod 7, given by . The inverse of that mapping is the discrete logarithm, i.e., the discrete logarithm of  is  means :

Congruence class mod 7 (written as smallest positive integer) Congruence class mod 7 (written as smallest magnitude integer) Discrete logarithm to base 3, written as integer mod 6 Is it a primitive root mod 7 (if and only if the discrete log is relatively prime to 6)? Is it s quadratic residue or nonresidue mod 7 (residue if discrete log is even, nonresidue if odd)
1 1 0 No quadratic residue
2 2 2 No quadratic residue
3 3 1 Yes quadratic nonresidue
4 -3 4 No quadratic residue
5 -2 5 Yes quadratic nonresidue
6 -1 3 No quadratic nonresidue

Alternatively, we could take discrete logs to base 5, which is the other primitive root. This is simply the negative of the other discrete log:

Congruence class mod 7 (written as smallest positive integer) Congruence class mod 7 (written as smallest magnitude integer) Discrete logarithm to base 5, written as integer mod 6 Is it a primitive root mod 7 (if and only if the discrete log is relatively prime to 6)? Is it s quadratic residue or nonresidue mod 7 (residue if discrete log is even, nonresidue if odd)
1 1 0 No quadratic residue
2 2 4 No quadratic residue
3 3 5 Yes quadratic nonresidue
4 -3 2 No quadratic residue
5 -2 1 Yes quadratic nonresidue
6 -1 3 No quadratic nonresidue

### Primitive roots

The number of primitive roots equals the number of generators of the additive group of integers modulo 6 (= 7 - 1) which is the Euler totient function of 6, which is 2. If  is a primitive root, the primitive roots are  and .

Explicitly, the primitive roots are 3 and 5 (= -2).

Of the six congruence classes of invertible elements mod 7, three are quadratic residues and three are quadratic nonresidues. If  is a primitive root, the quadratic nonresidues are , and the quadratic residues are . Alternatively, we can obtain the quadratic residues by taking the congruence classes of .

Explicitly, the quadratic residues are 1,2,4 and the quadratic nonresidues are 3,5,6.

Fill this in later

### Condition for 7 to be a quadratic residue modulo a prime

Whether or not 7 is a quadratic residue modulo a given prime depends only on the congruence class of that prime modulo 28. This follows from quadratic reciprocity (applicable only to odd primes, but the case of 2 is isolated), which states that:



We note that  depends only on the congruence class of  mod 7, and the right side depends only on the congruence class of  mod 4. Thus,  depends only on the congruence class of  mod 28.

The full list is below:

Fill this in later

By Dirichlet's theorem on arithmetic progressions, we thus find that there are infinitely many primes  for which 7 is a quadratic residue and infinitely many primes  for which 7 is a quadratic nonresidue.

## Curiosities

### Constructibility of regular 7-gon

7 is the smallest prime  for which the regular -gon is not constructible by straightedge and compass. In fact, it is the smallest natural number  for which the regular -gon is not constructible by straightedge and compass. This can be seen by noting that the cyclotomic extension of adjoining 7th roots is a degree six Galois extension and cannot be expressed using successive quadratic extensions.

### Significance of 10 being a primitive root

If 10 is a primitive root modulo a prime , then the prime  is a full reptend prime in base 10, i.e., the decimal expansion of  has a repeating block of the maximum possible length . The condition holds for  (note that  and 3 is a primitive root), and the corresponding decimal expansion of 1/7 is:



The corresponding number:



has the property that it is a cyclic number, i.e., its product with any of the numbers from 1 to 6 is obtained by cyclically permuting its digits.