Multiplicative group of integers modulo n

In modular arithmetic, the integers coprime (relatively prime) to n from the set ${\displaystyle \{0,1,\dots ,n-1\}}$ of n non-negative integers form a group under multiplication modulo n, called the multiplicative group of integers modulo n. Equivalently, the elements of this group can be thought of as the congruence classes, also known as residues modulo n, that are coprime to n. Hence another name is the group of primitive residue classes modulo n. In the theory of rings, a branch of abstract algebra, it is described as the group of units of the ring of integers modulo n. Here units refers to elements with a multiplicative inverse, which in this ring are exactly those coprime to n.

This group, usually denoted ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$, is fundamental in number theory. It has found applications in cryptography, integer factorization, and primality testing. It is an abelian, finite group whose order is given by Euler's totient function: ${\displaystyle |(\mathbb {Z} /n\mathbb {Z} )^{\times }|=\varphi (n).}$ For prime n the group is cyclic and in general the structure is easy to describe, though even for prime n no general formula for finding generators is known.

Group axioms

It is a straightforward exercise to show that, under multiplication, the set of congruence classes modulo n that are coprime to n satisfy the axioms for an abelian group.

Indeed, a is coprime to n if and only if gcd(a, n) = 1. Integers in the same congruence class ab (mod n) satisfy gcd(a, n) = gcd(b, n), hence one is coprime to n if and only if the other is. Thus the notion of congruence classes modulo n that are coprime to n is well-defined.

Since gcd(a, n) = 1 and gcd(b, n) = 1 implies gcd(ab, n) = 1, the set of classes coprime to n is closed under multiplication.

Integer multiplication respects the congruence classes, that is, aa' and bb' (mod n) implies aba'b' (mod n). This implies that the multiplication is associative, commutative, and that the class of 1 is the unique multiplicative identity.

Finally, given a, the multiplicative inverse of a modulo n is an integer x satisfying ax ≡ 1 (mod n). It exists precisely when a is coprime to n, because in that case gcd(a, n) = 1 and by Bézout's lemma there are integers x and y satisfying ax + ny = 1. Notice that the equation ax + ny = 1 implies that x is coprime to n, so the multiplicative inverse belongs to the group.

Notation

The set of (congruence classes of) integers modulo n with the operations of addition and multiplication is a ring. It is denoted ${\displaystyle \mathbb {Z} /n\mathbb {Z} }$  or  ${\displaystyle \mathbb {Z} /(n)}$  (the notation refers to taking the quotient of integers modulo the ideal ${\displaystyle n\mathbb {Z} }$ or ${\displaystyle (n)}$ consisting of the multiples of n). Outside of number theory the simpler notation ${\displaystyle \mathbb {Z} _{n}}$ is often used, though it can be confused with the p-adic integers when n is a prime number.

The multiplicative group of integers modulo n, which is the group of units in this ring, may be written as (depending on the author) ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times },}$   ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{*},}$   ${\displaystyle \mathrm {U} (\mathbb {Z} /n\mathbb {Z} ),}$   ${\displaystyle \mathrm {E} (\mathbb {Z} /n\mathbb {Z} )}$   (for German Einheit, which translates as unit), ${\displaystyle \mathbb {Z} _{n}^{*}}$, or similar notations. This article uses ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }.}$

The notation ${\displaystyle \mathrm {C} _{n}}$ refers to the cyclic group of order n. It is isomorphic to the group of integers modulo n under addition. Note that ${\displaystyle \mathbb {Z} /n\mathbb {Z} }$ or ${\displaystyle \mathbb {Z} _{n}}$ may also refer to the group under addition. For example, the multiplicative group ${\displaystyle (\mathbb {Z} /p\mathbb {Z} )^{\times }}$ for a prime p is cyclic and hence isomorphic to the additive group ${\displaystyle \mathbb {Z} /(p-1)\mathbb {Z} }$, but the isomorphism is not obvious.

Structure

The order of the multiplicative group of integers modulo n is the number of integers in ${\displaystyle \{0,1,\dots ,n-1\}}$ coprime to n. It is given by Euler's totient function: ${\displaystyle |(\mathbb {Z} /n\mathbb {Z} )^{\times }|=\varphi (n)}$ (sequence A000010 in the OEIS). For prime p, ${\displaystyle \varphi (p)=p-1}$.

Cyclic case

The group ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ is cyclic if and only if n is 1, 2, 4, pk or 2pk, where p is an odd prime and k > 0. For all other values of n the group is not cyclic.[1][2][3] This was first proved by Gauss.[4]

This means that for these n:

${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }\cong \mathrm {C} _{\phi (n)},}$ where ${\displaystyle \phi (p^{k})=\phi (2p^{k})=p^{k}-p^{k-1}.}$

By definition, the group is cyclic if and only if it has a generator g, that is, the powers ${\displaystyle g^{0},g^{1},g^{2},\dots ,}$ give all possible residues modulo n coprime to n (the first ${\displaystyle \phi (n)}$ powers ${\displaystyle g^{0},\dots ,g^{\phi (n)-1}}$ give each exactly once). A generator of ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ is called a primitive root modulo n.[5] If there is any generator, then there are ${\displaystyle \phi (\phi (n))}$ of them.

Powers of 2

Modulo 1 any two integers are congruent, i.e., there is only one congruence class, [0], coprime to 1. Therefore, ${\displaystyle (\mathbb {Z} /1\,\mathbb {Z} )^{\times }\cong \mathrm {C} _{1}}$ is the trivial group with φ(1) = 1 element. Because of its trivial nature, the case of congruences modulo 1 is generally ignored and some authors choose not to include the case of n = 1 in theorem statements.

Modulo 2 there is only one coprime congruence class, [1], so ${\displaystyle (\mathbb {Z} /2\mathbb {Z} )^{\times }\cong \mathrm {C} _{1}}$ is the trivial group.

Modulo 4 there are two coprime congruence classes, [1] and [3], so ${\displaystyle (\mathbb {Z} /4\mathbb {Z} )^{\times }\cong \mathrm {C} _{2},}$ the cyclic group with two elements.

Modulo 8 there are four coprime congruence classes, [1], [3], [5] and [7]. The square of each of these is 1, so ${\displaystyle (\mathbb {Z} /8\mathbb {Z} )^{\times }\cong \mathrm {C} _{2}\times \mathrm {C} _{2},}$ the Klein four-group.

Modulo 16 there are eight coprime congruence classes [1], [3], [5], [7], [9], [11], [13] and [15]. ${\displaystyle \{\pm 1,\pm 7\}\cong \mathrm {C} _{2}\times \mathrm {C} _{2},}$ is the 2-torsion subgroup (i.e., the square of each element is 1), so ${\displaystyle (\mathbb {Z} /16\mathbb {Z} )^{\times }}$ is not cyclic. The powers of 3, ${\displaystyle \{1,3,9,11\}}$ are a subgroup of order 4, as are the powers of 5, ${\displaystyle \{1,5,9,13\}.}$   Thus ${\displaystyle (\mathbb {Z} /16\mathbb {Z} )^{\times }\cong \mathrm {C} _{2}\times \mathrm {C} _{4}.}$

The pattern shown by 8 and 16 holds[6] for higher powers 2k, k > 2: ${\displaystyle \{\pm 1,2^{k-1}\pm 1\}\cong \mathrm {C} _{2}\times \mathrm {C} _{2},}$ is the 2-torsion subgroup (so ${\displaystyle (\mathbb {Z} /2^{k}\mathbb {Z} )^{\times }}$ is not cyclic) and the powers of 3 are a cyclic subgroup of order 2k 2, so ${\displaystyle (\mathbb {Z} /2^{k}\mathbb {Z} )^{\times }\cong \mathrm {C} _{2}\times \mathrm {C} _{2^{k-2}}.}$

General composite numbers

By the fundamental theorem of finite abelian groups, the group ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ is isomorphic to a direct product of cyclic groups of prime power orders.

More specifically, the Chinese remainder theorem[7] says that if ${\displaystyle \;\;n=p_{1}^{k_{1}}p_{2}^{k_{2}}p_{3}^{k_{3}}\dots ,\;}$ then the ring ${\displaystyle \mathbb {Z} /n\mathbb {Z} }$ is the direct product of the rings corresponding to each of its prime power factors:

${\displaystyle \mathbb {Z} /n\mathbb {Z} \cong \mathbb {Z} /{p_{1}^{k_{1}}}\mathbb {Z} \;\times \;\mathbb {Z} /{p_{2}^{k_{2}}}\mathbb {Z} \;\times \;\mathbb {Z} /{p_{3}^{k_{3}}}\mathbb {Z} \dots \;\;}$

Similarly, the group of units ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ is the direct product of the groups corresponding to each of the prime power factors:

${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }\cong (\mathbb {Z} /{p_{1}^{k_{1}}}\mathbb {Z} )^{\times }\times (\mathbb {Z} /{p_{2}^{k_{2}}}\mathbb {Z} )^{\times }\times (\mathbb {Z} /{p_{3}^{k_{3}}}\mathbb {Z} )^{\times }\dots \;.}$

For each odd prime power ${\displaystyle p^{k}}$ the corresponding factor ${\displaystyle (\mathbb {Z} /{p^{k}}\mathbb {Z} )^{\times }}$ is the cyclic group of order ${\displaystyle \phi (p^{k})=p^{k}-p^{k-1}}$, which may further factor into cyclic groups of prime-power orders. For powers of 2 the factor ${\displaystyle (\mathbb {Z} /{2^{k}}\mathbb {Z} )^{\times }}$ is not cyclic unless k = 0, 1, 2, but factors into cyclic groups as described above.

The order of the group ${\displaystyle \phi (n)}$ is the product of the orders of the cyclic groups in the direct product. The exponent of the group, that is, the least common multiple of the orders in the cyclic groups, is given by the Carmichael function ${\displaystyle \lambda (n)}$ (sequence A002322 in the OEIS). In other words, ${\displaystyle \lambda (n)}$ is the smallest number such that for each a coprime to n, ${\displaystyle a^{\lambda (n)}\equiv 1{\pmod {n}}}$ holds. It divides ${\displaystyle \phi (n)}$ and is equal to it if and only if the group is cyclic.

Subgroup of false witnesses

If n is composite, there exists a subgroup of the multiplicative group, called the "group of false witnesses", in which the elements, when raised to the power n − 1, are congruent to 1 modulo n (since the residue 1, to any power, is congruent to 1 modulo n, the set of such elements is nonempty).[8] One could say, because of Fermat's Little Theorem, that such residues are "false positives" or "false witnesses" for the primality of n. The number 2 is the residue most often used in this basic primality check, hence 341 = 11 × 31 is famous since 2340 is congruent to 1 modulo 341, and 341 is the smallest such composite number (with respect to 2). For 341, the false witnesses subgroup contains 100 residues and so is of index 3 inside the 300 element multiplicative group mod 341.

Examples

n = 9

The smallest example with a nontrivial subgroup of false witnesses is 9 = 3 × 3. There are 6 residues coprime to 9: 1, 2, 4, 5, 7, 8. Since 8 is congruent to −1 modulo 9, it follows that 88 is congruent to 1 modulo 9. So 1 and 8 are false positives for the "primality" of 9 (since 9 is not actually prime). These are in fact the only ones, so the subgroup {1,8} is the subgroup of false witnesses. The same argument shows that n − 1 is a "false witness" for any odd composite n.

n = 91

For n = 91 (= 7 x 13), there are ${\displaystyle \varphi (91)=72}$ residues coprime to 91, half of them (i.e., 36 of them) are false witnesses of 91, namely 1, 3, 4, 9, 10, 12, 16, 17, 22, 23, 25, 27, 29, 30, 36, 38, 40, 43, 48, 51, 53, 55, 61, 62, 64, 66, 68, 69, 74, 75, 79, 81, 82, 87, 88, and 90, since for these values of x, x90 is congruent to 1 mod 91.

n = 561

n = 561 (= 3 x 11 x 17) is a Carmichael number, thus s560 is congruent to 1 modulo 561 for any integer s coprime to 561. The subgroup of false witnesses is, in this case, not proper; it is the entire group of multiplicative units modulo 561, which consists of 320 residues.

Examples

This table shows the cyclic decomposition of ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ and a generating set for n ≤ 128. The generating sets are not unique; e.g., modulo 16 both {15, 3} and {15, 5} will work, in the list, we list the smallest values, (thus, for n with primitive root, we list the smallest primitive root modulo n) for example, for modulo 12, we list {5, 7} instead of {5, 11} or {7, 11} .

For example, we take n = 20. ${\displaystyle \varphi (20)=8}$ means that the order of ${\displaystyle (\mathbb {Z} /20\mathbb {Z} )^{\times }}$ is 8 (i.e., there are 8 numbers less than 20 and coprime to it); ${\displaystyle \lambda (20)=4}$ that is, the fourth power of any number coprime to 20 is congruent to 1 (mod 20); and as for the generators, 19 has (element) order 2, 3 has (element) order 4, and every member of ${\displaystyle (\mathbb {Z} /20\mathbb {Z} )^{\times }}$ is of the form 19a × 3b, where a is 0 or 1 and b is 0, 1, 2, or 3.

The powers of 19 are {±1} and the powers of 3 are {3, 9, 7, 1}. The latter and their negatives modulo 20, {17, 11, 13, 19} are all the numbers less than 20 and coprime to it. That the order of 19 is 2 and the order of 3 is 4 implies that the fourth power of every member of ${\displaystyle (\mathbb {Z} /20\mathbb {Z} )^{\times }}$ is congruent to 1 (mod 20).

Smallest primitive root mod n are (0 if no root exists)

0, 1, 2, 3, 2, 5, 3, 0, 2, 3, 2, 0, 2, 3, 0, 0, 3, 5, 2, 0, 0, 7, 5, 0, 2, 7, 2, 0, 2, 0, 3, 0, 0, 3, 0, 0, 2, 3, 0, 0, 6, 0, 3, 0, 0, 5, 5, 0, 3, 3, 0, 0, 2, 5, 0, 0, 0, 3, 2, 0, 2, 3, 0, 0, 0, 0, 2, 0, 0, 0, 7, 0, 5, 5, 0, 0, 0, 0, 3, 0, 2, 7, 2, 0, 0, 3, 0, 0, 3, 0, ... (sequence A046145 in the OEIS)

Numbers of the elements in a minimal generating set of mod n are

0, 0, 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 2, 1, 1, 1, 2, 2, 1, 1, 3, 1, 1, 1, 2, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 2, 3, 1, 2, 1, 2, 2, 1, 1, 3, 1, 1, 2, 2, 1, 1, 2, 3, 2, 1, 1, 3, 1, 1, 2, 2, 2, 2, 1, 2, 2, 2, 1, 3, 1, 1, 2, 2, 2, 2, 1, 3, 1, 1, 1, 3, 2, 1, 2, 3, 1, 2, ... (sequence A046072 in the OEIS)
${\displaystyle n\;}$ ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ ${\displaystyle \varphi (n)}$ ${\displaystyle \lambda (n)\;}$ Generating set ${\displaystyle n\;}$ ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ ${\displaystyle \varphi (n)}$ ${\displaystyle \lambda (n)\;}$ Generating set ${\displaystyle n\;}$ ${\displaystyle (\mathbb {Z} /n\mathbb {Z} )^{\times }}$ ${\displaystyle \varphi (n)}$ ${\displaystyle \lambda (n)\;}$ Generating set ${\displaystyle n\;}$ C1 1 1 0 C2×C10 20 10 2, 10 C4×C12 48 12 2, 12 C96 96 96 5 C1 1 1 1 C16 16 16 3 C2×C10 20 10 5, 7 C42 42 42 3 C2 2 2 2 C2×C12 24 12 2, 6 C66 66 66 2 C2×C30 60 30 2, 5 C2 2 2 3 C2×C6 12 6 5, 19 C2×C16 32 16 3, 67 C2×C20 40 20 3, 99 C4 4 4 2 C36 36 36 2 C2×C22 44 22 2, 68 C100 100 100 2 C2 2 2 5 C18 18 18 3 C2×C12 24 12 3, 69 C2×C16 32 16 5, 101 C6 6 6 3 C2×C12 24 12 2, 38 C70 70 70 7 C102 102 102 5 C2×C2 4 2 3, 5 C2×C2×C4 16 4 3, 11, 39 C2×C2×C6 24 6 5, 17, 19 C2×C2×C12 48 12 3, 5, 103 C6 6 6 2 C40 40 40 6 C72 72 72 5 C2×C2×C12 48 12 2, 29, 41 C4 4 4 3 C2×C6 12 6 5, 13 C36 36 36 5 C52 52 52 3 C10 10 10 2 C42 42 42 3 C2×C20 40 20 2, 74 C106 106 106 2 C2×C2 4 2 5, 7 C2×C10 20 10 3, 43 C2×C18 36 18 3, 37 C2×C18 36 18 5, 107 C12 12 12 2 C2×C12 24 12 2, 44 C2×C30 60 30 2, 76 C108 108 108 6 C6 6 6 3 C22 22 22 5 C2×C12 24 12 5, 7 C2×C20 40 20 3, 109 C2×C4 8 4 2, 14 C46 46 46 5 C78 78 78 3 C2×C36 72 36 2, 110 C2×C4 8 4 3, 15 C2×C2×C4 16 4 5, 7, 47 C2×C4×C4 32 4 3, 7, 79 C2×C2×C12 48 12 3, 5, 111 C16 16 16 3 C42 42 42 3 C54 54 54 2 C112 112 112 3 C6 6 6 5 C20 20 20 3 C40 40 40 7 C2×C18 36 18 5, 37 C18 18 18 2 C2×C16 32 16 5, 50 C82 82 82 2 C2×C44 88 44 2, 114 C2×C4 8 4 3, 19 C2×C12 24 12 7, 51 C2×C2×C6 24 6 5, 11, 13 C2×C28 56 28 3, 115 C2×C6 12 6 2, 20 C52 52 52 2 C4×C16 64 16 2, 3 C6×C12 72 12 2, 17 C10 10 10 7 C18 18 18 5 C42 42 42 3 C58 58 58 11 C22 22 22 5 C2×C20 40 20 2, 21 C2×C28 56 28 2, 86 C2×C48 96 48 3, 118 C2×C2×C2 8 2 5, 7, 13 C2×C2×C6 24 6 3, 13, 29 C2×C2×C10 40 10 3, 5, 7 C2×C2×C2×C4 32 4 7, 11, 19, 29 C20 20 20 2 C2×C18 36 18 2, 20 C88 88 88 3 C110 110 110 2 C12 12 12 7 C28 28 28 3 C2×C12 24 12 7, 11 C60 60 60 7 C18 18 18 2 C58 58 58 2 C6×C12 72 12 2, 3 C2×C40 80 40 7, 83 C2×C6 12 6 3, 13 C2×C2×C4 16 4 7, 11, 19 C2×C22 44 22 3, 91 C2×C30 60 30 3, 61 C28 28 28 2 C60 60 60 2 C2×C30 60 30 11, 61 C100 100 100 2 C2×C4 8 4 7, 11 C30 30 30 3 C46 46 46 5 C6×C6 36 6 5, 13 C30 30 30 3 C6×C6 36 6 2, 5 C2×C36 72 36 2, 94 C126 126 126 3 C2×C8 16 8 3, 31 C2×C16 32 16 3, 63 C2×C2×C8 32 8 5, 17, 31 C2×C32 64 32 3, 127

Notes

1. (Vinogradov 2003, pp. 105–121, § VI PRIMITIVE ROOTS AND INDICES)
2. (Gauss & Clarke 1986, arts. 52–56, 82–891)
3. (Vinogradov 2003, p. 106)
4. (Gauss & Clarke 1986, arts. 90–91)
5. Riesel covers all of this. (Riesel 1994, pp. 267–275)
6. Erdős, Paul; Pomerance, Carl (1986). "On the number of false witnesses for a composite number". Math. Comput. 46 (173): 259–279. doi:10.1090/s0025-5718-1986-0815848-x. Zbl 0586.10003.

References

The Disquisitiones Arithmeticae has been translated from Gauss's Ciceronian Latin into English and German. The German edition includes all of his papers on number theory: all the proofs of quadratic reciprocity, the determination of the sign of the Gauss sum, the investigations into biquadratic reciprocity, and unpublished notes.

• Gauss, Carl Friedrich; Clarke, Arthur A. (translator into English) (1986), Disquisitiones Arithmeticae (Second, corrected edition), New York: Springer, ISBN 978-0-387-96254-2
• Gauss, Carl Friedrich; Maser, H. (translator into German) (1965), Untersuchungen uber hohere Arithmetik (Disquisitiones Arithemeticae & other papers on number theory) (Second edition), New York: Chelsea, ISBN 978-0-8284-0191-3
• Riesel, Hans (1994), Prime Numbers and Computer Methods for Factorization (second edition), Boston: Birkhäuser, ISBN 978-0-8176-3743-9
• Vinogradov, I. M. (2003), "§ VI PRIMITIVE ROOTS AND INDICES", Elements of Number Theory, Mineola, NY: Dover Publications, pp. 105–121, ISBN 978-0-486-49530-9