Cayley-Hamilton Theorem/Finitely Generated Module

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

Let $M$ be a finitely generated $A$-module.

Let $\mathfrak a$ be an ideal of $A$.

Let $\phi$ be an endomorphism of $M$ such that $\phi \left({M}\right) \subseteq \mathfrak a M$.

Then $\phi$ satisfies an equation of the form:


 * $\phi^n + a_{n-1} \phi^{n-1} + \cdots + a_1 \phi + a_0 = 0$

with the $a_i \in \mathfrak a$.

Proof
Let $m_1, \ldots, m_n$ be a generating set for $M$.

Then for each $i$, $\phi \left({m_i}\right) \in \mathfrak a M$, say:


 * $\displaystyle \phi \left({m_i}\right) = \sum_{j \mathop = 1}^n a_j m_j$

for $i = 1, \ldots, n$.

Thus for each $i$:


 * $(1): \quad \displaystyle \sum_{j \mathop = 1}^n \left[{\delta_{ij} \phi - a_{ij} }\right] m_i = 0$

where $\delta_{ij}$ is the Kronecker delta.

Now let $\Delta$ be the matrix defined as:
 * $\Delta := \left({\phi \delta_{ij} - a_{ij} }\right)$

Let $\operatorname{adj} \left({\Delta}\right)$ be the adjugate matrix of $\Delta$.

Recall Cramer's Rule:

Multiplying through by $\operatorname{adj} \left({\Delta}\right)$ in $(1)$ and applying Cramer's Rule:


 * $\displaystyle \sum_{j \mathop = 1}^n \det \left({\Delta}\right) m_i = 0$

Therefore $\det \left({\Delta}\right)$ annihilates each $m_i$ and is the zero endomorphism of $M$.

But $\det \left({\phi \delta_{ij} - a_{ij}}\right)$ is a monic polynomial in $\phi$ with coefficients in $\mathfrak a$.

Thus we have an equation of the required form.