This article is about a particular natural number.|View all articles on particular natural numbers
- 1 Summary
- 2 Structure of integers mod 7
- 3 Quadratic theory
- 4 Curiosities
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
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)|
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)|
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).
Quadratic residues and nonresidues
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.
Adjacent quadratic residues
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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:
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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.
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.