Relative Matrix of Composition of Linear Transformations/Proof 1
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Theorem
Let $R$ be a ring with unity.
Let $M, N, P$ be free $R$-modules of finite dimension $m, n, p > 0$ respectively.
Let $\AA, \BB, \CC$ be ordered bases of $M, N, P$.
Let $f: M \to N$ and $g : N \to P$ be linear transformations, and $g \circ f$ be their composition.
Let $\mathbf M_{f, \BB, \AA}$ and $\mathbf M_{g, \CC, \BB}$ be their matrices relative to $\AA, \BB$ and $\BB, \CC$ respectively.
Then the matrix of $g \circ f$ relative to $\AA$ and $\CC$ is:
- $\mathbf M_{g \mathop \circ f, \CC, \AA} = \mathbf M_{g, \CC, \BB} \cdot \mathbf M_{f, \BB, \AA}$
Proof
Let $m \in M$, and $\sqbrk m_\AA$ be its coordinate vector with respect to $\AA$.
On the one hand:
\(\ds \sqbrk {\map g {\map f m} }_\CC\) | \(=\) | \(\ds \mathbf M_{g \mathop \circ f, \CC, \AA} \cdot \sqbrk m_\AA\) | Change of Coordinate Vectors Under Linear Mapping applied to $g \circ f$ |
On the other hand:
\(\ds \sqbrk {\map g {\map f m} }_\CC\) | \(=\) | \(\ds \mathbf M_{g, \CC, \BB} \cdot \sqbrk {\map f m}_\BB\) | Change of Coordinate Vectors Under Linear Mapping applied to $g$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \mathbf M_{g, \CC, \BB} \cdot \mathbf M_{f, \BB, \AA} \cdot \sqbrk m_\AA\) | Change of Coordinate Vectors Under Linear Mapping applied to $f$ |
Thus:
- $\forall m \in M: \paren {\mathbf M_{g \mathop \circ f, \CC, \AA} - \mathbf M_{g, \CC, \BB} \cdot \mathbf M_{f, \BB, \AA} } \cdot \sqbrk m_\AA = 0$
The result follows.
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$\blacksquare$