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Question:

$V$ is a linear space over $\mathbb{C}$, $A$ is a linear transformation on $V$ which is diagonalizable (i.e. there is a basis of $V$ consists of eigenvectors of $A$).

If $W\subseteq V$ is an invariant subspace of $A$ (i.e. $AW\subseteq W$), show that $A|_W$ is also diagonalizable.

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I tried to suppose $A$ have different eigenvalues $\lambda_1,\ldots,\lambda_m$, with $V_i=\{v\in V: Av=\lambda_i v\}$.

Then we can write

$$V=V_1\oplus\cdots\oplus V_m,$$

But I'm not sure whether it is true that

$$W=(W\cap V_1)\oplus\cdots\oplus (W\cap V_m),$$

if it is true, then we're done, but it may be wrong.

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@NGY: It's not a counterexample. –  Arturo Magidin Sep 6 '11 at 17:02
    
@Arturo, thanks, so do you think my hypothesis is right? –  NGY Sep 6 '11 at 17:06
    
@NGY: Since $W$ is in fact diagonalizable, and any eigenvector of $A|_W$ is necessarily an eigenvector of $A$, yes, it's true: you know $W=W_1\oplus \cdots \oplus W_m$ (some of them possibly trivial). Find a basis of eigenvectors for each $W_i$, then extend to a basis of eigenvectors of the corresponding $V_i$. But I think proving it ex nihilo will be difficult. –  Arturo Magidin Sep 6 '11 at 17:10
    
The desired result can also be phrased as: let $A$ be a linear operator on a finite dimensional vector space (over any field). If $A$ is diagonalizable, then every invariant subspace is a direct sum of one-dimensional invariant subspaces. As Arturo says though, it doesn't seem straightforward to prove this directly. His nice answer using canonical forms looks like the best way to go. –  Pete L. Clark Sep 6 '11 at 17:55

3 Answers 3

up vote 8 down vote accepted

Theorem. A linear transformation is diagonalizable if and only if its minimal polynomial splits and has no repeated factors.

Proof. This follows by examining the Jordan canonical form, since the largest power of $(x-\lambda)$ that divides the minimal polynomial is equal to the size of the largest block of corresponding to $\lambda$ of the Jordan canonical form of the linear transformation. (Use the fact that every irreducible factor of the characteristic polynomial divides the minimal polynomial, and that the characteristic polynomial must split for the linear transformation to be diagonalizable to argue that you can restrict yourself to linear transformations with Jordan canonical forms). QED

Theorem. Let $A$ be a linear transformation on $V$, and let $W\subseteq V$ be an $A$-invariant subspace. Then the minimal polynomial of the restriction of $A$ to $W$, $A|_{W}$, divides the minimal polynomial of $A$.

Proof. Let $B=A|_{W}$, and let $\mu(x)$ be the minimal polynomial of $A$. Since $\mu(A)=0$ on all of $V$, the restriction of $\mu(A)$ to $W$ is $0$; but $\mu(A)|_{W} = \mu(A|_{W}) = \mu(B)$. Since $\mu(B)=0$, then the minimal polynomial of $B$ divides $\mu(x)$. QED

Corollary. If $A$ is diagonalizable, and $W$ is $A$-invariant, then the restriction of $A$ to $W$ is diagonalizable.

Proof. The minimal polynomial of $A$ splits and has no repeated factors; since the minimal polynomial of $A|_W$ divides a polynomial that splits and has no repeated factors, it follows that it itself has no repeated factors and splits. Thus, the restriction of $A$ to $W%$ is also diagonalizable. QED

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This theorem is true for arbitrary V (over an arbitrary field F).

We can prove the following theorem: If $v_1 + v_2 + \cdots + v_k \in W$ and each of the $v_i$ are eigenvectors of $A$ with distinct eigenvalues, each of the $v_i$ lie in $W$.

Proof: Proceed by induction. If $k = 1$ there is nothing to prove. Otherwise, let $w = v_1 + \cdots + v_k$, and $\lambda_i$ be the eigenvalue corresponding to $v_i$. Then:

$$Aw - \lambda_1w = (\lambda_2 - \lambda_1)v_2 + \cdots + (\lambda_k - \lambda_1)v_k \in W$$

By induction, $(\lambda_i - \lambda_1)v_i \in W$, and since the eigenvalues $\lambda_i$ are distinct, $v_i \in W$ for $2 \leq i \leq k$, then we also have $v_1 \in W \quad \square$

Now each $w \in W$ can be written as a finite sum of nonzero eigenvectors of $A$ with distinct eigenvalues, and by the theorem these eigenvectors lie in $W$.Then we have $W = \bigoplus_{\lambda \in F}(W \cap V_{\lambda})$ as desired (where $V_{\lambda} = \{v \in V\mid Av = \lambda v\}$.

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+1! Wonderful! ${}$ –  Pierre-Yves Gaillard Nov 2 '11 at 7:10

Here is a minor variation of Zorn's very nice argument. I'll use Zorn's notation:

Let $w=v_1 + v_2 + \cdots + v_k$ be in $W$, each $v_i$ being a $\lambda_i$-eigenvector of $A$, and the $\lambda_i$ being distinct.

It suffices to check that each $v_i$ is in $W$.

But this is clear since

$$v_i=\prod_{j\neq i}\ \frac{A-\lambda_j\,I}{\lambda_i-\lambda_j}\quad.$$

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