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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 -indexed chain) is assumed only for the converse and is available under the Axiom of Choice (The Axiom of Choice).
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 , write (1) every submodule is finitely generated, (2) ACC, and (3) every nonempty set of submodules has a maximal member. The implications and are choice-free; assumes DC, under which the three conditions are equivalent. (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; 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 carries both conditions to . At this gives the forward implication, without any 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 maximal proper submodule by the nonempty maximal condition of [L2], using DC. Define a relation on the submodules of 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 ; it strictly decreases until it reaches zero and is then constant. These are the precise choice uses in the converse.
The chain of step 1.2 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
- Composition series and length of a module
- Finite generation, ACC, and maximal-condition characterizations of Noetherian modules
- DCC and minimal-condition characterizations of Artinian modules
- Noetherian and Artinian conditions are each exact in short exact sequences
- The axiom of dependent choice: a relation in which every element is related to something admits an $\mathbb{N}$-indexed chain
- The Axiom of Choice
Used by
- Semisimple rings are left and right Noetherian and Artinian Corollary
- False statement: every module has a composition series False statement
- A normal p-subgroup acts trivially on every simple module in characteristic p Proposition
- A commutative ring is Artinian exactly when it has finite length as a module over itself Theorem
Dependency tree · two levels
19 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Arvind Nair, Algebra I, Lecture 5 (standard reference, not scraped)