Lagrange's theorem (group theory)

File:Left cosets of Z 2 in Z 8.svg

G is the group $\mathbb{\mathbb{Z}}/8\mathbb{\mathbb{Z}}$, the integers mod 8 under addition. The subgroup H contains only 0 and 4, and is isomorphic to $\mathbb{\mathbb{Z}}/2\mathbb{\mathbb{Z}}$. There are four left cosets of H: H itself, 1+H, 2+H, and 3+H (written using additive notation since this is an additive group). Together they partition the entire group G into equal-size, non-overlapping sets. Thus the index [G : H] is 4.

In the mathematical field of group theory, Lagrange's theorem states that if Template:Mvar is a subgroup of any finite group Template:Mvar, then $|H|$ is a divisor of $|G|$. That is, the order (number of elements) of every subgroup divides the order of the whole group.

The theorem is named after Joseph-Louis Lagrange. The following variant states that for a subgroup $H$ of a finite group $G$, not only is $|G|/|H|$ an integer, but its value is the index $[G:H]$, defined as the number of left cosets of $H$ in $G$. Template:Math theorem This variant holds even if $G$ is infinite, provided that $|G|$, $|H|$, and $[G:H]$ are interpreted as cardinal numbers.

Proof

The left cosets of Template:Mvar in Template:Mvar are the equivalence classes of a certain equivalence relation on Template:Mvar: specifically, call Template:Mvar and Template:Mvar in Template:Mvar equivalent if there exists Template:Mvar in Template:Mvar such that x = yh. Therefore, the set of left cosets forms a partition of Template:Mvar. Each left coset aH has the same cardinality as Template:Mvar because $x\mapsto ax$ defines a bijection $H\to aH$ (the inverse is $y\mapsto a^{-1}y$). The number of left cosets is the index [G : H]. By the previous three sentences,

$\left|G\right|=[G:H]\cdot \left|H\right|.$

Extension

Lagrange's theorem can be extended to the equation of indices between three subgroups of Template:Mvar. Template:Math theorem Template:Math proof If we take K = Template:Mset (Template:Mvar is the identity element of Template:Mvar), then [G : Template:Mset] = |G| and [H : Template:Mset] = |H|. Therefore, we can recover the original equation |G| = [G : H] |H|.

Applications

A consequence of the theorem is that the order of any element Template:Mvar of a finite group (i.e. the smallest positive integer number Template:Mvar with Template:Mvar^{Template:Mvar} = Template:Mvar, where Template:Mvar is the identity element of the group) divides the order of that group, since the order of Template:Mvar is equal to the order of the cyclic subgroup generated by Template:Mvar. If the group has Template:Mvar elements, it follows

$a^n=e\text{.}$

This can be used to prove Fermat's little theorem and its generalization, Euler's theorem. These special cases were known long before the general theorem was proved.

The theorem also shows that any group of prime order is cyclic and simple, since the subgroup generated by any non-identity element must be the whole group itself.

Lagrange's theorem can also be used to show that there are infinitely many primes: suppose there were a largest prime $p$. Any prime divisor $q$ of the Mersenne number $2^p-1$ satisfies $2^p\equiv 1(\mathrm{mod}q)$ (see modular arithmetic), meaning that the order of $2$ in the multiplicative group $(\mathbb{\mathbb{Z}}/q\mathbb{\mathbb{Z}}{)}^{*}$ is $p$. By Lagrange's theorem, the order of $2$ must divide the order of $(\mathbb{\mathbb{Z}}/q\mathbb{\mathbb{Z}}{)}^{*}$, which is $q-1$. So $p$ divides $q-1$, giving $p<q$, contradicting the assumption that $p$ is the largest prime.

Existence of subgroups of given order

Lagrange's theorem raises the converse question as to whether every divisor of the order of a group is the order of some subgroup. This does not hold in general: given a finite group G and a divisor d of |G|, there does not necessarily exist a subgroup of G with order d. The smallest example is A₄ (the alternating group of degree 4), which has 12 elements but no subgroup of order 6.

A "Converse of Lagrange's Theorem" (CLT) group is a finite group with the property that for every divisor of the order of the group, there is a subgroup of that order. It is known that a CLT group must be solvable and that every supersolvable group is a CLT group. However, there exist solvable groups that are not CLT (for example, A₄) and CLT groups that are not supersolvable (for example, S₄, the symmetric group of degree 4).

There are partial converses to Lagrange's theorem. For general groups, Cauchy's theorem guarantees the existence of an element, and hence of a cyclic subgroup, of order any prime dividing the group order. Sylow's theorem extends this to the existence of a subgroup of order equal to the maximal power of any prime dividing the group order. For solvable groups, Hall's theorems assert the existence of a subgroup of order equal to any unitary divisor of the group order (that is, a divisor coprime to its cofactor).

Counterexample of the converse of Lagrange's theorem

The converse of Lagrange's theorem states that if Template:Mvar is a divisor of the order of a group Template:Mvar, then there exists a subgroup Template:Mvar where |H| = d.

We will examine the alternating group A₄, the set of even permutations as the subgroup of the Symmetric group S₄.

A₄ = Template:Mset.

|A₄| = 12 so the divisors are 1, 2, 3, 4, 6, 12. Assume to the contrary that there exists a subgroup Template:Mvar in A₄ with |H| = 6.

Let Template:Mvar be the non-cyclic subgroup of A₄ called the Klein four-group.

V = Template:Mset.

Let K = H ⋂ V. Since both Template:Mvar and Template:Mvar are subgroups of A₄, Template:Mvar is also a subgroup of A₄.

From Lagrange's theorem, the order of Template:Mvar must divide both 6 and 4, the orders of Template:Mvar and Template:Mvar respectively. The only two positive integers that divide both 6 and 4 are 1 and 2. So |K| = 1 or 2.

Assume |K| = 1, then K = Template:Mset. If Template:Mvar does not share any elements with Template:Mvar, then the 5 elements in Template:Mvar besides the Identity element Template:Mvar must be of the form (a b c) where a, b, c are distinct elements in Template:Mset.

Since any element of the form (a b c) squared is (a c b), and (a b c)(a c b) = e, any element of Template:Mvar in the form (a b c) must be paired with its inverse. Specifically, the remaining 5 elements of Template:Mvar must come from distinct pairs of elements in A₄ that are not in Template:Mvar. This is impossible since pairs of elements must be even and cannot total up to 5 elements. Thus, the assumptions that |K| = 1 is wrong, so |K| = 2.

Then, K = Template:Mset where v ∈ V, Template:Mvar must be in the form (a b)(c d) where Template:Mvar are distinct elements of Template:Mset. The other four elements in Template:Mvar are cycles of length 3.

Note that the cosets generated by a subgroup of a group form a partition of the group. The cosets generated by a specific subgroup are either identical to each other or disjoint. The index of a subgroup in a group [A₄ : H] = |A₄|/|H| is the number of cosets generated by that subgroup. Since |A₄| = 12 and |H| = 6, Template:Mvar will generate two left cosets, one that is equal to Template:Mvar and another, Template:Mvar, that is of length 6 and includes all the elements in A₄ not in Template:Mvar.

Since there are only 2 distinct cosets generated by Template:Mvar, then Template:Mvar must be normal. Because of that, H = gHg⁻¹ (∀g ∈ A₄). In particular, this is true for g = (a b c) ∈ A₄. Since H = gHg⁻¹, gvg⁻¹ ∈ H.

Without loss of generality, assume that a = 1, b = 2, c = 3, d = 4. Then g = (1 2 3), v = (1 2)(3 4), g⁻¹ = (1 3 2), gv = (1 3 4), gvg⁻¹ = (1 4)(2 3). Transforming back, we get gvg⁻¹ = (a d)(b c). Because Template:Mvar contains all disjoint transpositions in A₄, gvg⁻¹ ∈ V. Hence, gvg⁻¹ ∈ H ⋂ V = K.

Since gvg⁻¹ ≠ v, we have demonstrated that there is a third element in Template:Mvar. But earlier we assumed that |K| = 2, so we have a contradiction.

Therefore, our original assumption that there is a subgroup of order 6 is not true and consequently there is no subgroup of order 6 in A₄ and the converse of Lagrange's theorem is not necessarily true. Q.E.D.

History

Lagrange himself did not prove the theorem in its general form. He stated, in his article Réflexions sur la résolution algébrique des équations, that if a polynomial in Template:Mvar variables has its variables permuted in all n! ways, the number of different polynomials that are obtained is always a factor of n!. (For example, if the variables Template:Mvar, Template:Mvar, and Template:Mvar are permuted in all 6 possible ways in the polynomial x + y − z then we get a total of 3 different polynomials: x + y − z, x + z − y, and y + z − x. Note that 3 is a factor of 6.) The number of such polynomials is the index in the symmetric group S_n of the subgroup Template:Mvar of permutations that preserve the polynomial. (For the example of x + y − z, the subgroup Template:Mvar in S₃ contains the identity and the transposition (x y).) So the size of Template:Mvar divides n!. With the later development of abstract groups, this result of Lagrange on polynomials was recognized to extend to the general theorem about finite groups which now bears his name.

In his Disquisitiones Arithmeticae in 1801, Carl Friedrich Gauss proved Lagrange's theorem for the special case of $(\mathbb{\mathbb{Z}}/p\mathbb{\mathbb{Z}}{)}^{*}$, the multiplicative group of nonzero integers modulo Template:Mvar, where Template:Mvar is a prime. In 1844, Augustin-Louis Cauchy proved Lagrange's theorem for the symmetric group S_n.

Camille Jordan finally proved Lagrange's theorem for the case of any permutation group in 1861.