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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.

[L1]

A composition series of a left R-module M is a finite chain 0=M0<M1<<Mn=M whose factors Mi/Mi1 are simple. If such a series exists, the length R(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, the following are equivalent: every submodule is finitely generated; every ascending chain of submodules stabilizes; and every nonempty family of submodules has a maximal member. The implication from ACC to the maximal condition uses dependent choice; the other displayed implications are choice-free. (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 0NMQ0, 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
1.1

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 Mi/Mi1 has only the submodules 0 and itself, so every chain of its submodules stabilizes and it too satisfies both conditions. Applying [L4] to the short exact sequence 0Mi1MiMi/Mi10 carries both conditions from Mi1 and the simple factor to Mi. At i=n this gives the forward implication, which uses no choice principle.

L1L4givenalgebra
2.1

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 nonempty family of proper submodules, which by the maximal condition of [L2] has a maximal member — a maximal proper submodule of N. Dependent choice applied to this relation, starting at M, yields a chain M=N0>N1> in which Nk+1 is a maximal proper submodule of Nk for as long as Nk0.

step 1.1L2L4givenalgebra
3.1

The chain of step 2.1 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.

L1L3step 2.1givenalgebra

Depends on

Used by

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 22 results over 9 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.

Sources