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A module has a composition series if and only if it is Noetherian and Artinian, the converse using dependent choice

Statement

A module with a composition series is both Noetherian and Artinian. Conversely, assuming dependent choice, a module that is both Noetherian and Artinian has a composition series. The zero module has the empty composition series. See Composition series and length of a module.

Facts & Assumptions

Given: The hypotheses and objects in the Statement. Dependent choice (The axiom of dependent choice: a relation in which every element is related to something admits an N-indexed chain) is assumed only for the converse and is available under the Axiom of Choice (The Axiom of Choice).

[L1]

A composition series of a left R-module M is a finite chain 0=M0<M1<⋯<Mn=M whose factors Mi/Mi−1 are simple. If such a series exists, the length ℓR(M) is its number n of factors; thm-jordan-holder-theorem-for-modules proves independence of the chosen series. The zero module has the empty series and length 0. (Composition series and length of a module).

[L2]

For a left R-module M, write (1) every submodule is finitely generated, (2) ACC, and (3) every nonempty set of submodules has a maximal member. The implications 1⇒2 and 3⇒1 are choice-free; 2⇒3 assumes DC, under which the three conditions are equivalent. (Finite generation, ACC, and maximal-condition characterizations of Noetherian modules).

[L3]

For a left R-module M, DCC is equivalent to the condition that every nonempty family of submodules has a minimal member. The implication from DCC to the minimal condition uses dependent choice. (DCC and minimal-condition characterizations of Artinian modules).

[L4]

In a short exact sequence 0→N→M→Q→0, the module M is Noetherian if and only if N and Q are Noetherian; the same equivalence holds with “Artinian” in place of “Noetherian”. (Noetherian and Artinian conditions are each exact in short exact sequences).

Proof

technique · direct
1.1L1L4givenalgebra

Let 0=M0<⋯<Mn=M be a composition series [L1] and induct on i that Mi is Noetherian and Artinian. The zero module M0 satisfies both conditions vacuously. A simple factor has only the submodules 0 and itself; any one nonzero element generates it, since its cyclic submodule is nonzero. Thus all its submodules are finitely generated and all descending chains stabilize. Applying [L4] to 0→Mi−1→Mi→Mi/Mi−1→0 carries both conditions to Mi. At i=n this gives the forward implication, without any choice principle.

1.2L2L4givenchoose

Conversely, assume dependent choice and let M be Noetherian and Artinian. Every submodule of M is Noetherian by [L4], so a nonzero submodule N has a maximal proper submodule by the nonempty maximal condition of [L2], using DC. Define a relation on the submodules of M by taking a maximal proper submodule at each nonzero term and letting zero be its own successor. This relation is serial. DC, available from the assumed AC, supplies a sequence starting at M; it strictly decreases until it reaches zero and is then constant. These are the precise choice uses in the converse.

2.1L1L3step 1.2givenalgebra∎

The chain of step 1.2 is strictly descending while its terms are nonzero, so the descending chain condition forces some Nr=0. Since Nk+1 is maximal proper in Nk, the quotient Nk/Nk+1 is nonzero and has no proper nonzero submodule, hence is simple. Reversing the chain gives 0=Nr<⋯<N0=M, a composition series. For M=0 the empty chain is already the required series, so no choice is consumed in that case. This proves the stated claim.

Depends on

Used by

Dependency tree · two levels

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Sources