# Dimension of Range and Null Space of Composition of Two Linear Maps

Question

Suppose $U$, $V$ and $W$ are finite-dimensional vector spaces. Let $\mathcal{L}(U,V)$ and $\mathcal{L}(V,W)$ be the vector spaces of all linear maps from $U$ into $V$ and from $V$ into $W$, respectively. Suppose $S \in \mathcal{L}(V,W)$ and $T \in \mathcal{L}(U,V)$. Then prove that

\begin{align} & 1. \, \text{dim null} S \circ T \le \text{dim null} S + \text{dim null} T \\ & 2. \, \text{dim range} S \circ T \le \text{min} \{ \text{dim range} S , \text{dim range} T \} \end{align}

My Thought

To prove the first one, I guess that writing the fundamental theorem of linear maps for $S \circ T$ may be a good start

\begin{align} \text{dim null} S \circ T &= \text{dim} U – \text{dim range} S \circ T \\ &= \text{dim null} T + \text{dim range} T – \text{dim range} S \circ T \\ &\le \text{dim null} T + \text{dim} V – \text{dim range} S \circ T \\ &= \text{dim null} T + \text{dim null} S + \text{dim range} S- \text{dim range} S \circ T \end{align}

So if I can prove that

$$\text{dim range} S- \text{dim range} S \circ T \le 0$$

then I am done but this does not seem to be true because it is easy to see that $\text{range} S \circ T \subseteq \text{range} S$ and hence
$\text{dim range} S \circ T \le \text{dim range} S$
. So I am stuck!
Also, I could observe that $\text{null} T \subseteq \text{null} S \circ T$ and hence
$\text{dim null} T \le \text{dim null} S \circ T$
but I don’t know how to use this!

#### Solutions Collecting From Web of "Dimension of Range and Null Space of Composition of Two Linear Maps"

Fo the first question, I would argue like this: let $K = \mathrm{null}(ST) = T^{-1}(\mathrm{null}(S))$, a subspace of $U$, and let $T|_{K} : K \to V$ be the restriction of $T$ to $K$. Because

\begin{align*}
\mathrm{null}(T|_{K}) \subset \mathrm{null}(T) && \text{and} &&\mathrm{ran}(T|_{K}) \subset \mathrm{null}(S)
\end{align*}

we get

\begin{align*}
\dim(\mathrm{null}(ST))
&= \dim(K) \\
&= \dim(\mathrm{null}(T|_{K})) + \dim(\mathrm{ran}(T|_{K})) \\
&\leq \dim(\mathrm{null}(T)) + \dim(\mathrm{null}(S))
\end{align*}

as desired.

For your second question, note what you need to do is prove (i) $\dim(\mathrm{ran}(ST))≤\dim(\mathrm{ran}(S))$ and (ii) $\dim(\mathrm{ran}(ST))≤\dim(\mathrm{ran}(T))$. I think you can probably see why (i) is true. For (ii), note that $ST$ has the same range as $S|_{\mathrm{ran}(T)}$, and that the range of a linear tranformation has equal or lesser dimension than the domain