Theorem 3.7. Let T: V → W and U: W – Z be linear transformations on finite-dimensional vector spaces V, W, and Z, and let A and B be matrices such that the product AB is defined. Then (a) rank(UT) < rank(U).

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Please see attached Linear Algebra problem.

The proof of Theorem 3.7 used matrix representations and transposition to show that rank(UT)≤rank(T)rank(UT)≤rank(T).

How to devise an alternative proof for this inequality, using ordered bases and the Rank-Nullity Theorem (but neither matrix representation nor transposition)?

### Theorem 3.7

**Statement:**
Let \( T: V \to W \) and \( U: W \to Z \) be linear transformations on finite-dimensional vector spaces \( V, W, \) and \( Z \), and let \( A \) and \( B \) be matrices such that the product \( AB \) is defined. Then
(a) \(\text{rank}(UT) \leq \text{rank}(U)\).

**Proof:**

(a) Clearly, \( R(T) \subseteq W \). Hence

\[
R(UT) = UT(V) = U(T(V)) = U(R(T)) \subseteq U(W) = R(U).
\]

Thus

\[
\text{rank}(UT) = \text{dim}(R(UT)) \leq \text{dim}(R(U)) = \text{rank}(U).
\]
Transcribed Image Text:### Theorem 3.7 **Statement:** Let \( T: V \to W \) and \( U: W \to Z \) be linear transformations on finite-dimensional vector spaces \( V, W, \) and \( Z \), and let \( A \) and \( B \) be matrices such that the product \( AB \) is defined. Then (a) \(\text{rank}(UT) \leq \text{rank}(U)\). **Proof:** (a) Clearly, \( R(T) \subseteq W \). Hence \[ R(UT) = UT(V) = U(T(V)) = U(R(T)) \subseteq U(W) = R(U). \] Thus \[ \text{rank}(UT) = \text{dim}(R(UT)) \leq \text{dim}(R(U)) = \text{rank}(U). \]
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