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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.
A composition series of a left -module is a finite chain whose factors are simple. If such a series exists, the length is its number of factors; thm-jordan-holder-theorem-for-modules proves independence of the chosen series. The zero module has the empty series and length . (Composition series and length of a module).
For a left -module , 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).
For a left -module , 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).
In a short exact sequence , the module is Noetherian if and only if and are Noetherian; the same equivalence holds with “Artinian” in place of “Noetherian”. (Noetherian and Artinian conditions are each exact in short exact sequences).
Proof
Let be a composition series [L1] and induct on that is Noetherian and Artinian. The zero module satisfies both conditions vacuously. A simple factor has only the submodules and itself, so every chain of its submodules stabilizes and it too satisfies both conditions. Applying [L4] to the short exact sequence carries both conditions from and the simple factor to . At this gives the forward implication, which uses no choice principle.
Conversely, assume dependent choice and let be Noetherian and Artinian. Every submodule of is Noetherian by [L4], so a nonzero submodule has a nonempty family of proper submodules, which by the maximal condition of [L2] has a maximal member — a maximal proper submodule of . Dependent choice applied to this relation, starting at , yields a chain in which is a maximal proper submodule of for as long as .
The chain of step 2.1 is strictly descending while its terms are nonzero, so the descending chain condition forces some . Since is maximal proper in , the quotient is nonzero and has no proper nonzero submodule, hence is simple. Reversing the chain gives , a composition series. For 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 · 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
- Arvind Nair, Algebra I, Lecture 5 (standard reference, not scraped)