Equivalence of Definitions of Noetherian Ring

Theorem
Let $A$ be a commutative ring with unity.

Definition 2 iff Definition 3
This follows by Increasing Sequence in Ordered Set Terminates iff Maximal Element.

Definition 2 implies Definition 1
Assume there is an ideal $I$ which is not finitely generated.

For any finite set $\left\{ {a_1, \dotsc, a_n}\right \}$ where $n \in \N$, the generated ideal is not equal to $I$.

Consider the chain:
 * $\left\langle{a_1}\right\rangle \subset \left\langle{a_1, a_2}\right\rangle \subset \cdots$

This chain does not satisfy the ascending chain condition (note that $I$ has infinitely many elements by assumption).

Definition 1 implies Definition 2
Let there be a chain of ideals $I_1 \subset I_2 \subset \cdots$.

Then $\displaystyle J = \bigcup_{n \mathop \ge 1} I_n$ is an ideal.

Let $J$ be finitely generated, by $\left\{ {b_1, \dotsc b_m }\right\}$ for some $m \in \N$.

As the chain is ascending, there exists an ideal such that:
 * $\left\{ {b_1, \dotsc b_m }\right\} \subset I_k$

for some $k \in \N$.

It follows that:
 * $I_k = \left\langle{b_1, \dotsc, b_m}\right\rangle$

Hence:
 * $\forall l \ge k: I_l = I_k$