# Why does the spectral norm equal the largest singular value?

This may be a trivial question yet I was unable to find an answer:

$$\left \| A \right \| _2=\sqrt{\lambda_{\text{max}}(A^{^*}A)}=\sigma_{\text{max}}(A)$$

where the spectral norm $\left \| A \right \| _2$ of a complex matrix $A$ is defined as $$\text{max} \left\{ \|Ax\|_2 : \|x\| = 1 \right\}$$

How does one prove the first and the second equality?

• What are your thoughts on the second equality? – Git Gud Nov 30 '13 at 11:35
• @GitGud Oh, does that mean singular values are defined in this way, i.e., the second equality is a definition? – mathemage Nov 30 '13 at 12:23
• It's not a definition. It's a direct consequence of the definition. Can you see this? – Git Gud Nov 30 '13 at 12:26
• @GitGud One normally can't if one does not know about the Courant-Fischer's characterisation. – Algebraic Pavel Dec 1 '13 at 2:37

Put $$B=A^*A$$ which is a Hermitian matrix.

As a linear transformation of Euclidean vector space $$E$$ is Hermite iff there exists an orthonormal basis of E consisting of all the eigenvectors of $$B$$

Let $$\lambda_1,...,\lambda_n$$ be the eigenvalues of $$B$$ and $$\left \{ e_1,...e_n \right \}$$ be an orthonormal basis of $$E$$

Let $$x=a_1e_1+...+a_ne_n$$

we have $$\left \| x \right \|=\left \langle \sum_{i=1}^{n}a_ie_i,\sum_{i=1}^{n}a_ie_i \right \rangle^{1/2} =\sqrt{\sum_{i=1}^{n}a_i^{2}}$$,

$$Bx=B\left ( \sum_{i=1}^{n}a_ie_i \right )=\sum_{i=1}^{n}a_iB(e_i)=\sum_{i=1}^{n}\lambda_ia_ie_i$$

Denote $$\lambda_{j_{0}}$$ to be the largest eigenvalue of $$B$$.

Therefore,

$$\left \| Ax \right \|=\sqrt{\left \langle Ax,Ax \right \rangle}=\sqrt{\left \langle x,A^*Ax \right \rangle}=\sqrt{\left \langle x,Bx \right \rangle}=\sqrt{\left \langle \sum_{i=1}^{n}a_ie_i,\sum_{i=1}^{n}\lambda_ia_ie_i \right \rangle}=\sqrt{\sum_{i=1}^{n}a_i\overline{\lambda_ia_i}} \leq \underset{1\leq j\leq n}{\max}\sqrt{\left |\lambda_j \right |} \times (\left \| x \right \|)$$

So, if $$\left \| A \right \|$$ = $$\max \left\{ \|Ax\| : \|x\| = 1 \right\}$$ then $$\left \| A \right \|\leq \underset{1\leq j\leq n}\max\sqrt{\left |\lambda_j \right |}$$ (1)

Consider: $$x_0=e_{j_{0}}$$ $$\Rightarrow \left \| x_0 \right \|=1$$ so that $$\left \| A \right \|^2 \geq \left \langle x_0,Bx_0 \right \rangle=\left \langle e_{j_0},B(e_{j_0}) \right \rangle=\left \langle e_{j_0},\lambda_{j_0} e_{j_0} \right \rangle = \lambda_{j_0}$$ (2)

Combining (1) and (2) gives us $$\left \| A \right \|= \underset{1\leq j\leq n}{\max}\sqrt{\left | \lambda_{j} \right |}$$ where $$\lambda_j$$ is the eigenvalue of $$B=A^*A$$

Conclusion: $$\left \| A \right \| _2=\sqrt{\lambda_{\text{max}}(A^{^*}A)}=\sigma_{\text{max}}(A)$$

• Why is $A^*A$ a unitary matrix? For $A$ as a zero matrix or a general real diagonal matrix, this doesn't have to be unitary, right? – mathemage Nov 30 '13 at 16:29
• That's my mistake. That should be Hermite. Got fixed! – An Khuong Doan Nov 30 '13 at 16:58
• I think I understood the proof except for one part: in the "iff" statement, why does the reverse implication hold? For any unitary $U$ and diagonal non-Hermitian $\Lambda$ the matrix $B = U^\dagger \Lambda U$ has eigenvectors forming orthonormal basis (namely, columns of $U$). However, $B$ is not Hermitian since $\Lambda$ is not. This is a counterexample, right? – mathemage Dec 1 '13 at 19:09
• And (2) is still blur to me: how can we say $$\left \| A \right \| \geq \left \langle x,Bx \right \rangle=\left \langle e_{j_0},B(e_{j_0}) \right \rangle$$ – sleeve chen Jan 21 '15 at 0:01
• @sleevechen for (1) we can say it because $\left \| A \right \|$ is the maximum of all the elements in the set, and it was just shown that each element is $\leq$ the largest singular value times $\left \| x \right \| = 1$. For (2), since $\left \| A \right \|$ is the maximum, it must be $\geq$ than any particular element, namely $\left \| A {e_j}_0 \right \|=\left \| A{e_j}_0\right \|=\left \langle A{e_j}_0,A{e_j}_0 \right \rangle=\left \langle {e_j}_0,A^*A{e_j}_0 \right \rangle=\left \langle {e_j}_0,B{e_j}_0 \right \rangle$ – ignoramus Jun 8 '16 at 11:40

First of all, \begin{align*}\sup_{\|x\|_2 =1}\|Ax\|_2 & = \sup_{\|x\|_2 =1}\|U\Sigma V^Tx\|_2 = \sup_{\|x\|_2 =1}\|\Sigma V^Tx\|_2\end{align*} since $$U$$ is unitary, that is, $$\|Ux_0\|_2^2 = x_0^TU^TUx_0 = x_0^Tx_0 = \|x_0\|_2^2$$, for some vector $$x_0$$.

Then let $$y = V^Tx$$. By the same argument above, $$\|y\|_2 = \|V^Tx\|_2 = \|x\|_2 = 1$$ since $$V$$ is unitary. $$\sup_{\|x\|_2 =1}\|\Sigma V^Tx\|_2 = \sup_{\|y\|_2 =1}\|\Sigma y\|_2$$ Since $$\Sigma = \mbox{diag}(\sigma_1, \cdots, \sigma_n)$$, where $$\sigma_1$$ is the largest singular value. The max for the above, $$\sigma_1$$, is attained when $$y = (1,\cdots,0)^T$$. You can find the max by, for example, solving the above using a Lagrange Multiplier.