Equivalence of Definitions of Generator of Unitary Module
Theorem
Let $R$ be a ring with unity.
Let $M$ be a unitary $R$-module.
Let $S \subseteq M$ be a subset.
The following definitions of the concept of Generator of Unitary Module are equivalent:
Definition by linear combinations
$S$ is a generator of $M$ if and only if every element of $M$ is a linear combination of elements of $S$.
Definition by intersection of submodules
$S$ is a generator of $M$ if and only if $M$ is the submodule generated by $S$.
That is, $M$ is equal to the intersection of all submodules of $M$ containing $S$.
Definition by proper submodules
$S$ is a generator of $M$ if and only if $M$ has no proper submodule containing $S$.
Proof
Definition by linear combinations implies Definition by intersection of submodules
Definition by intersection of submodules implies Definition by linear combinations
By definition of submodule, it follows that $M$ can be considered as a submodule of itself.
From Equivalence of Definitions of Generated Submodule over Ring with Unity, it follows that:
- the set of all linear combinations of elements of $S$
is equal to:
- the intersection of all submodules of $M$ containing $S$.
Hence, the two definitions of generator of unitary module by linear combinations and by intersection of submodules are equivalent.
$\Box$
Definition by intersection of submodules implies Definition by proper submodules
Definition by proper submodules implies Definition by intersection of submodules
By definition of module, it follows that $M$ is an $R$-module.
The equivalence of the definitions now follows from Equivalence of Definitions of Generator of Module.
$\blacksquare$