# Prove that if $A$ is normal, then eigenvectors corresponding to distinct eigenvalues are necessarily orthogonal (alternative proof)

The problem statement is as follows:

Prove that for a normal matrix $A$, eigenvectors corresponding to different eigenvalues are necessarily orthogonal.

I can certainly prove that this is the case, using the spectral theorem. The gist of my proof is presented below.

If possible, I would like to find a simpler proof. I was hoping that there might be some sort of manipulation along these lines, noting that $$\langle Av_1,A v_2\rangle = \langle v_1,A^*Av_2\rangle = \langle v_1,AA^*v_2\rangle = \langle A^* v_1,A^* v_2 \rangle$$

Any ideas here would be appreciated.

## My proof:

Let $\{v_{\lambda,i}\}$ be an orthonormal basis of eigenvectors (as guaranteed by the spectral theorem) such that $$A v_{\lambda,i} = \lambda v_{\lambda,i}$$ Let $v_1,\lambda_1$ and $v_2,\lambda_2$ be eigenpairs with $\lambda_1 \neq \lambda_2$. We may write $v_1 = \sum_{i,\lambda}a_{i,\lambda}v_{i,\lambda} .$ We then have $$0 = Av_1 - \lambda_1 v_1 = \sum_{i,\lambda}(\lambda - \lambda_1)a_{i,\lambda}v_{i,\lambda}$$ So that $a_{i,\lambda} = 0$ when $\lambda \neq \lambda_1$. Similarly, we may write $v_2 = \sum_{i,\lambda}b_{i,\lambda}v_{i,\lambda}$, and note that $b_{i,\lambda} = 0$ when $\lambda \neq \lambda_2$. From there, we have $$\langle v_1,v_2 \rangle = \sum_{i,\lambda}a_{i,\lambda}b_{i,\lambda}$$ the above must be zero since for each pair $i,\lambda$, either $a_{i,\lambda}=0$ or $b_{i,\lambda} = 0$.

• "Let $v_{\lambda,i}$ be an orthonormal basis of eigenvectors..." I'd guess that this might be something very close to what would need to be proved. May 3, 2014 at 2:13
• @PavelJiranek I was worried that I had used circular logic at some point. However, the existence of such a basis (i.e. the spectral theorem) comes directly from the Schur triangularization theorem, which says nothing about normal matrices in particular. May 3, 2014 at 19:11

Assume $\;\lambda\neq \mu\;$ and

$$\begin{cases}Av=\lambda v\;\,\implies\; A^*v=\overline \lambda v\\{}\\Aw=\mu w\implies A^*w=\overline\mu w\end{cases}$$

From this we get:

$$\begin{cases}\langle v,Aw\rangle=\langle v,\mu w\rangle=\overline\mu\langle v,w\rangle\\{}\\ \langle v,Aw\rangle=\langle A^*v,w\rangle=\langle\overline\lambda v,w\rangle=\overline\lambda\langle v,w\rangle \end{cases}$$

and since $\;\overline\mu\neq\overline\lambda\;$ , we get $\;\langle v,w\rangle =0\;$

Question: Where did we use normality in the above?

• How de we know that $v$ is also an eigenvector for $A^*$? May 2, 2014 at 22:55
• From the other two answers this follows, of course, using that $A$ is normal. But can you show it directly? May 2, 2014 at 22:57
• Can someone give me a clue as to where you used normality in the above? Jul 9, 2018 at 1:19
• If p(x,y) is a polynomial then since A is normal, the matrix B := p(A,A*) is also normal. Normality of B implies that B and B* have the same kernel, using the identity <Bv,Bv> = <B* v,B* v> derived by the OP. By taking p(x,y) := x-lambda we get the implications asserted in the answer above. Jun 20, 2019 at 15:14
• Check this answer Dec 7, 2020 at 18:08

Specializing your identity to $v_1=v_2=v$, we get $\|Av\|=\|A^*v\|$. Hence $\ker A=\ker A^*$. Recalling that $\ker A^* = (\operatorname{ran} A)^\perp$ for general $A$, we conclude that the kernel and range of a normal matrix are mutually orthogonal.

It remains to apply the above conclusion to $A-\lambda I$ where $\lambda$ is an eigenvalue of $A$.

• Neat! Although DonAntonio's proof is more along the lines I was thinking of, this is a nice perspective. May 2, 2014 at 23:16

The answers to this question are very good but they are glossing over the fact that if $$A \in \mathbb C^{n \times n}$$ is normal then $$Ax = \lambda x \implies A^*x = \bar \lambda x$$. I think it's helpful to just spell out the whole thing explicitly.

Lemma: If $$M \in \mathbb C^{n \times n}$$ is normal, then $$M$$ and $$M^*$$ have the same null space.

Proof: Let $$M \in \mathbb C^{n \times n}$$ be a normal matrix. Then \begin{align} &M^*x = 0 \\ \iff & \| M^* x \|^2 = 0 \\ \iff & \langle M^*x, M^* x \rangle = 0 \\ \iff & \langle M M^* x, x \rangle = 0 \\ \iff & \langle M^* M x, x \rangle = 0 \\ \iff & \langle Mx, Mx \rangle = 0 \\ \iff & \| Mx \|^2 = 0 \\ \iff & Mx = 0. \end{align}

Lemma: If $$A \in \mathbb C^{n \times n}$$ is normal then $$Ax = \lambda x \implies A^* x = \bar \lambda x$$.

Proof: Suppose that $$A \in \mathbb C^{n \times n}$$ is normal and $$Ax = \lambda x$$. So $$x$$ is a null vector of $$M = A - \lambda I$$. Note that $$M$$ is normal, as you can check by expanding both $$M^* M$$ and $$M M^*$$. By the above lemma, $$x$$ is also a null vector of $$M^* = A^* - \bar \lambda I$$. Thus $$A^*x = \bar \lambda x$$.

Theorem: If $$x$$ and $$y$$ are eigenvectors corresponding to distinct eigenvalues of a normal matrix $$A \in \mathbb C^{n \times n}$$, then $$\langle x, y \rangle = 0$$.

Proof: Let $$A \in \mathbb C^{n \times n}$$ be a normal matrix, and suppose that $$x$$ and $$y$$ are eigenvectors of $$A$$ corresponding to distinct eigenvalues $$\lambda$$ and $$\gamma$$, respectively. Note that $$\langle x, Ay \rangle = \langle x, \gamma y \rangle = \bar \gamma \langle x, y \rangle.$$ On the other hand, $$\langle x, Ay \rangle = \langle A^* x, y \rangle = \langle \bar \lambda x, y \rangle = \bar \lambda \langle x, y \rangle.$$ So we find that $$\bar \gamma \langle x, y \rangle = \bar \lambda \langle x, y \rangle$$, which implies that $$\langle x, y \rangle = 0$$.

• Thank you for this very helpful answer! Shouldn't the last member of the last two displayed expressions have unconjugated gamma and lambda? May 1, 2022 at 22:57
• @RodolfoOviedo I’m using the convention that if $x, y \in \mathbb C^n$ then $\langle x, y \rangle = \sum_i x_i \bar{y_i}$. With this convention, I think the way that I wrote it is correct. Sometimes people (especially physicists, I believe) define $\langle x, y \rangle$ to be $\sum_i \bar{x_i} y_i$. Perhaps you have that definition in mind. May 2, 2022 at 0:14
• Thanks for your answer! After the second sentence of the proof of the second lemma, I would add: Because $A$ is normal, so is $M$, as you can check by expanding both $M^*M$ and $MM^*$. May 2, 2022 at 22:45
• Am I right? If so, should you or I edit the answer? May 2, 2022 at 22:52
• @RodolfoOviedo I just made the change you suggested, thanks. May 2, 2022 at 23:39

linear_algebra_done_right This is from linear algebra done right. btw, it's the greatest book about linear algebra I've ever seen!

Proof: Suppose $$\alpha,\beta$$ are distinct eigenvalues of $$T$$, with corresponding eigenvectors $$u,v$$. Thus, $$Tu = \alpha u$$ and $$Tv = \beta v$$. From 7.21 we have $$T^*v = \bar \beta v.$$ Thus, \begin{align} (\alpha - \beta)\langle u,v \rangle &= \langle \alpha u,v \rangle - \langle u, \bar \beta v \rangle \\ &= \langle Tu,v \rangle - \langle u, T^*v \rangle \\ & = 0. \end{align} Because $$\alpha \neq \beta$$, the equation above implies $$\langle u,v \rangle = 0$$. Thus, $$u$$ and $$v$$ are orthogonal, as desired.

• LADR should have been the first place that I looked. Thanks for your answer Nov 24, 2020 at 14:22

I try to give another simple proof to $$T^*v=\bar{\lambda}v ~\text{ if }~ Tv=\lambda v$$ where $T$ is a normal operator on a Hilbert space $H$.

Suppose $V=\ker(T-\lambda I)$. Since $T^*$ communicate with $T$, $$T^*V\subset V.$$ Because $$\langle v,T^*v\rangle =\langle Tv,v\rangle =\langle \lambda v,v\rangle=\langle v,\bar{\lambda}v\rangle ~~\forall v \in V,$$ $\langle u,T^*v\rangle =\langle u, \bar{\lambda}v\rangle ~\forall u,v\in V$ by polarisation identity, and thus $T^*v=\bar{\lambda}v.$

REMARK: Let $\sigma:V\times V\to W$ be a sesquilinear form, where $V$ and $W$ are linear vector spaces over $\mathbb{C}$. The follwing formula is called Polarisation Identity : $$\sigma(u,v)=\sum_{k=0}^3 i^k\sigma(u+i^k v, u+i^kv).$$

• I don't really see how this answer addresses the question being asked. Also, the question being asked was answered 3 years ago. Jun 12, 2017 at 15:01
• @Omnomnomnom This is why $Av=\lambda v\Rightarrow A^*v=\bar{\lambda}v$ using no spectral theorem which is the point to the question I think. Jun 13, 2017 at 2:37
• That is not the point to the question, actually. The point is to show that eigenvectors corresponding to different eigenvalues are necessarily orthogonal. Jun 13, 2017 at 2:58
• Oh, sorry to disturb you. As I give some additional remarks to the right answer you have accepted. Jun 13, 2017 at 3:04

I'll use the notation from Introduction to Linear Algebra by Strang. Let $$A^H$$ be the transpose conjugate of $$A$$, and suppose $$A$$ is normal, i.e. $$AA^H = A^H A$$.

Examine an eigenpair $$Ax = \lambda x$$. Multiply by $$A^H$$ and we have $$A^H Ax = AA^H x = \lambda A^H x$$. Assume only one $$x$$ with eigenvalue $$\lambda$$ (see note at end), so here we know that $$A^H x$$ is a multiple of $$x$$, i.e. $$A^H x = c x$$. Multiply by $$x^H$$ and we have $$c = \bar \lambda$$, i.e. the conjugate of $$\lambda$$.

Now consider another eigenpair $$A y = \mu y$$ with $$\mu \neq \lambda$$. From the result in the previous paragraph, we have $$y^H A = \mu y^H$$. Now multiply by $$x$$ and we obtain $$\mu y^H x = \lambda y^H x$$, which leads to $$y^H x = 0$$.

Note: if there are multiple $$x$$ with eigenvalue $$\lambda$$, then choose $$x$$ to be the linear combination that is also an eigenvector of $$A^H$$. Such a choice is always be possible given that there's at least one eigenvector with eigenvalue $$\lambda$$ (proof is left as an exercise for the reader).

A normal matrix is unitarily similar to diagonal matrix.

$$A = UDU^{-1}$$ where $U$ is Unitary matrix.

Eigen decompositions tells that $U$ is a matrix composed of columns which are eigenvectors of $A$. And matrix $D$ is Diagonal matrix with eigenvalues on diagonal.

Property: Columns of Unitary matrix are orthogonal.

So, columns of $U$ (which are eigenvectors of $A$) are orthogonal.