The Structure of Finite Abelian Groups

We prove the fundamental theorem of finite abelian groups: every finite abelian group is isomorphic to a direct product of cyclic groups. The two canonical forms --- elementary divisors and invariant factors --- are developed, and the classification procedure is illustrated with explicit examples.

Folio Official

March 1, 2026

1 Goal

Our objective is to determine the structure of every finite abelian group completely. Specifically, we will show that every finite abelian group is isomorphic to a direct product of cyclic groups, and that this decomposition is essentially unique.

2 Basic Properties of Cyclic Groups

Proposition 1.

g cd (m, n) = 1

, then

Z / mn Z ≅ Z / m Z \times Z / n Z

Proof.

Define

φ : Z / mn Z \to Z / m Z \times Z / n Z

φ (a) = (a mod m, a mod n)

. This is a homomorphism. If

φ (a) = (0, 0)

, then

m ∣ a

and

n ∣ a

; since

g cd (m, n) = 1

, we get

mn ∣ a

, so

a = 0

Z / mn Z

. Thus

φ

is injective, and since

∣ Z / mn Z ∣ = mn = ∣ Z / m Z \times Z / n Z ∣

, it is an isomorphism. □

Corollary 2.

n = p_{1}^{a_{1}} p_{2}^{a_{2}} \dots p_{r}^{a_{r}}

is the prime factorization of

n

, then

Z / n Z ≅ Z / p_{1}^{a_{1}} Z \times Z / p_{2}^{a_{2}} Z \times \dots \times Z / p_{r}^{a_{r}} Z .

3 Decomposition of $p$ -Groups

Lemma 3.

Let

G

be a finite abelian

p

-group (i.e.,

∣ G ∣ = p^{n}

). If

g \in G

is an element of maximal order, then

⟨ g ⟩

is a direct factor of

G

: there exists a subgroup

H

such that

G = ⟨ g ⟩ \times H

The proof of this lemma proceeds by induction on $∣ G ∣$ . The key idea is as follows.

Proof.

Let

∣ g ∣ = p^{k}

. We induct on

∣ G ∣ = p^{n}

. If

k = n

, then

G = ⟨ g ⟩

and the result is trivial. Suppose

k < n

Let

π : G \to G / ⟨ g ⟩

be the natural projection. By induction,

G / ⟨ g ⟩

decomposes as a direct product of cyclic groups.

For each cyclic direct factor

⟨ \overset{ˉ}{h} ⟩

G / ⟨ g ⟩

(where

\overset{ˉ}{h} = π (h)

), we can choose the lift

h

so that

ord (h) = ord (\overset{ˉ}{h})

; this is possible because

g

has maximal order. Repeating this process yields

G = ⟨ g ⟩ \times H

. □

Theorem 4 (Decomposition of finite abelian

p

-groups).

Every finite abelian

p

-group

G

is isomorphic to a direct product of cyclic

p

-groups:

G ≅ Z / p^{a_{1}} Z \times Z / p^{a_{2}} Z \times \dots \times Z / p^{a_{r}} Z, a_{1} \geq a_{2} \geq \dots \geq a_{r} \geq 1.

Proof.

By induction on

∣ G ∣

. The preceding lemma gives

G = ⟨ g ⟩ \times H

where

∣ g ∣ = p^{a_{1}}

and

∣ H ∣ < ∣ G ∣

. Applying the induction hypothesis to

H

yields

H ≅ Z / p^{a_{2}} Z \times \dots \times Z / p^{a_{r}} Z

. Since

g

has maximal order,

a_{1} \geq a_{2}

. □

4 Primary Decomposition

Theorem 5 (Primary decomposition).

Let

G

be a finite abelian group with

∣ G ∣ = p_{1}^{n_{1}} p_{2}^{n_{2}} \dots p_{r}^{n_{r}}

. Writing

G_{p_{i}}

for the Sylow

p_{i}

-subgroup of

G

G = G_{p_{1}} \times G_{p_{2}} \times \dots \times G_{p_{r}} .

Proof.

Since

G

is abelian, each Sylow subgroup

G_{p_{i}}

is unique, hence normal. We have

∣ G_{p_{i}} ∣ = p_{i}^{n_{i}}

. Since

g cd (p_{i}^{n_{i}}, \prod_{j \neq = i} p_{j}^{n_{j}}) = 1

, it follows that

G_{p_{i}} \cap \prod_{j \neq = i} G_{p_{j}} = {e}

. As

\prod_{i} ∣ G_{p_{i}} ∣ = ∣ G ∣

, we conclude

G = G_{p_{1}} \times \dots \times G_{p_{r}}

. □

5 The Fundamental Theorem of Finite Abelian Groups

Combining the results of the previous sections:

Theorem 6 (Fundamental theorem — elementary divisor form).

Every finite abelian group

G

is isomorphic to a direct product of cyclic groups:

G ≅ Z / p_{1}^{a_{1}} Z \times Z / p_{2}^{a_{2}} Z \times \dots \times Z / p_{s}^{a_{s}} Z

where the

p_{i}

are primes (not necessarily distinct) and

a_{i} \geq 1

. This decomposition is unique up to reordering. The prime powers

p_{1}^{a_{1}}, \dots, p_{s}^{a_{s}}

are called the elementary divisors of

G

Proof.

By the primary decomposition theorem,

G = G_{p_{1}} \times G_{p_{2}} \times \dots \times G_{p_{r}}

where each

G_{p_{i}}

is the Sylow

p_{i}

-subgroup. Each

G_{p_{i}}

is a finite abelian

p_{i}

-group, so by the decomposition theorem for

p

-groups,

G_{p_{i}} ≅ Z / p_{i}^{a_{i, 1}} Z \times Z / p_{i}^{a_{i, 2}} Z \times \dots \times Z / p_{i}^{a_{i, r_{i}}} Z .

Collecting these factors across all primes gives the desired decomposition. Uniqueness follows from the uniqueness of the decomposition of each

p

-component (proved below). □

Theorem 7 (Fundamental theorem — invariant factor form).

Every finite abelian group

G

can be written as

G ≅ Z / d_{1} Z \times Z / d_{2} Z \times \dots \times Z / d_{t} Z, d_{1} ∣ d_{2} ∣ \dots ∣ d_{t}, d_{i} \geq 2.

This decomposition is unique. The integers

d_{1}, \dots, d_{t}

are called the invariant factors of

G

Proof.

Starting from the elementary divisor decomposition, we construct the invariant factors as follows. For each prime

p

, list its elementary divisors in decreasing order:

p^{a_{1}} \geq p^{a_{2}} \geq \dots \geq p^{a_{r_{p}}}

where

a_{1} \geq a_{2} \geq \dots \geq a_{r_{p}} \geq 1

Set

t = max_{p} r_{p}

(the maximum number of factors across all primes). For

j = 1, \dots, t

, define

d_{j} = p \prod p^{a_{j} (p)}

where

a_{j} (p)

is the

j

-th exponent for the prime

p

(with

a_{j} (p) = 0

when

j > r_{p}

Since

a_{1} (p) \geq a_{2} (p) \geq \dots

for each

p

, we have

d_{1} ∣ d_{2} ∣ \dots ∣ d_{t}

. By the Chinese Remainder Theorem (

g cd (m, n) = 1

implies

Z / mn Z ≅ Z / m Z \times Z / n Z

Z / d_{j} Z ≅ p \prod Z / p^{a_{j} (p)} Z,

\prod_{j} Z / d_{j} Z

agrees with the original elementary divisor decomposition. Uniqueness follows from that of the elementary divisors. □

The elementary divisors and invariant factors carry the same information in different packaging. To pass from elementary divisors to invariant factors, one groups the prime powers by prime and multiplies them together in a specific pattern; the reverse conversion is obtained by factoring each invariant factor into prime powers.

6 Proof of Uniqueness

Theorem 8 (Uniqueness).

The elementary divisor decomposition of a finite abelian group is unique up to reordering.

Proof.

Let

p

be a prime. From the group

G

, one can read off intrinsic invariants using the subgroups

G [p] = {g \in G ∣ g^{p} = e}

and

G^{p} = {g^{p} ∣ g \in G}

G ≅ Z / p^{a_{1}} Z \times \dots \times Z / p^{a_{r}} Z

with

a_{1} \geq \dots \geq a_{r} \geq 1

, then

∣ G [p] ∣ = p^{r}

, which determines

r

(the number of cyclic factors in the

p

-component).

Moreover,

G / G^{p} ≅ (Z / p Z)^{r}

, and the Sylow

p

-subgroup of

G^{p}

is isomorphic to

Z / p^{a_{1} - 1} Z \times \dots \times Z / p^{a_{r} - 1} Z

(with factors of exponent

0

dropped). Iterating this procedure determines each

a_{i}

uniquely. □

7 Classification in Practice

Example 9 (Abelian groups of order

8

We have

∣ G ∣ = 8 = 2^{3}

. The partitions of

3

are

(3)

(2, 1)

, and

(1, 1, 1)

. The corresponding abelian groups are:

$Z /8 Z$ (elementary divisors ${8}$ , invariant factors ${8}$ ).
$Z /4 Z \times Z /2 Z$ (elementary divisors ${4, 2}$ , invariant factors ${2, 4}$ ).
$Z /2 Z \times Z /2 Z \times Z /2 Z$ (elementary divisors ${2, 2, 2}$ , invariant factors ${2, 2, 2}$ ).

Example 10 (Abelian groups of order

36

We have

36 = 2^{2} \cdot 3^{2}

. The partitions of

2

are

(2)

and

(1, 1)

, for both the

2

-part and the

3

-part. This gives

2 \times 2 = 4

groups:

$Z /4 Z \times Z /9 Z ≅ Z /36 Z$ (invariant factors ${36}$ ).
$Z /4 Z \times Z /3 Z \times Z /3 Z$ (invariant factors ${3, 12}$ ).
$Z /2 Z \times Z /2 Z \times Z /9 Z$ (invariant factors ${2, 18}$ ).
$Z /2 Z \times Z /2 Z \times Z /3 Z \times Z /3 Z ≅ Z /6 Z \times Z /6 Z$ (invariant factors ${6, 6}$ ).

Example 11 (Abelian groups of order

72

We have

72 = 2^{3} \cdot 3^{2}

. There are

3

partitions of

3

and

2

partitions of

2

, giving

3 \times 2 = 6

groups:

$Z /8 Z \times Z /9 Z ≅ Z /72 Z$ .
$Z /8 Z \times Z /3 Z \times Z /3 Z$ .
$Z /4 Z \times Z /2 Z \times Z /9 Z$ .
$Z /4 Z \times Z /2 Z \times Z /3 Z \times Z /3 Z$ .
$Z /2 Z \times Z /2 Z \times Z /2 Z \times Z /9 Z$ .
$Z /2 Z \times Z /2 Z \times Z /2 Z \times Z /3 Z \times Z /3 Z$ .

8 Exponent and Rank

Definition 12.

For a finite abelian group

G

The exponent $exp (G)$ is the least common multiple of the orders of all elements of $G$ . In the invariant factor form, $exp (G) = d_{t}$ (the largest invariant factor).
The rank of $G$ is the number of factors in the elementary divisor decomposition.

Proposition 13.

G

is cyclic

\Leftrightarrow

exp (G) = ∣ G ∣

\Leftrightarrow

G

has exactly one invariant factor.

9 Summary

Every finite abelian group decomposes as a direct product of cyclic groups (in both the elementary divisor form and the invariant factor form).
The decomposition is unique, providing a complete classification of finite abelian groups.
The classification reduces to the prime factorization of the group order: for each prime power, one enumerates partitions.
The theory of abelian groups is, in this sense, "complete"— a striking contrast to the vastly more complex structure theory of non-abelian groups.

Group Theory Algebra Textbook Finite Abelian Groups Structure Theorem

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1 フォロワー·105 記事

The Structure of Finite Abelian Groups

1 Goal

2 Basic Properties of Cyclic Groups

3 Decomposition of $p$ -Groups

4 Primary Decomposition

5 The Fundamental Theorem of Finite Abelian Groups

6 Proof of Uniqueness

7 Classification in Practice

8 Exponent and Rank

9 Summary

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1 Goal

2 Basic Properties of Cyclic Groups

3 Decomposition ofp-Groups

4 Primary Decomposition

5 The Fundamental Theorem of Finite Abelian Groups

6 Proof of Uniqueness

7 Classification in Practice

8 Exponent and Rank

9 Summary

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