# Do two matrices close in spectral norm have close traces?

Let $A_p$ and $B_p$ be a sequence of symmetric positive-definite $p\times p$ matrices such that $$\|A_p - B_p\|_2 \le 1/p.$$ Suppose further that the spectra of $A_p$ and $B_p$, for all $p$, is contained in a compact interval in $(0,\infty)$. And finally suppose that for all $p$, $\operatorname{tr}(B_p) = p.$

Intuitively, it seems that it should follow from this that $$\|pA_p/\operatorname{tr}(A_p) - B_p\|_2 \le const./p.$$

Does this in fact hold?

• I think in this case you have that all eigenvalues of $A_p$ are within $1/p$ of those of $B_p$, but I'm not entirely certain because they will typically not be simultaneously diagonalizable. Given that it is easy, since $pA_p/tr(A_p)$ will be equal to $A_p+O(1/p^2)$. I'm just not sure how to get that. – Ian Dec 29 '16 at 21:02
• I managed to solve it. The trick is to use your observation, Ian, and note that in general we have by the Weyl inequalities $\lambda_k(A)\le \lambda_k(B)+\|A-B\|_2$. – Lepidopterist Jan 2 '17 at 21:55

Yes, it does, and you don't need Weyl's inequality. Note that $$|\operatorname{tr}(A_p-B_p)|\le\sum_i|\lambda_i(A_p-B_p)|\le p\|A_p-B_p\|_2\le1.$$ Therefore $\operatorname{tr}(A_p)= p+\operatorname{tr}(A_p-B_p) \in [p-1,p+1]$ and $$\frac{-1}{p+1}=\frac{p}{p+1}-1\le\frac{p}{\operatorname{tr}(A_p)}-1\le\frac{p}{p-1}-1=\frac{1}{p-1}.$$ In turn, $\left|\frac{p}{\operatorname{tr}(A_p)}-1\right|=O(\frac1p)$. Since we have assumed that the spectrum of $B_p$ is confined inside a compact interval, we also have $\|B_p\|_2=O(1)$. Thus \begin{align*} \left\|\frac{p}{\operatorname{tr}(A_p)}A_p-B_p\right\|_2 &\le\frac{p}{\operatorname{tr}(A_p)}\|A_p-B_p\|_2 + \left|\frac{p}{\operatorname{tr}(A_p)}-1\right| \|B_p\|_2\\ &=O(1)O(\frac1p) + O(\frac1p)O(1) = O(\frac1p). \end{align*}

• Doesn't this equivalence of norms only hold for finite $p$? Does the big-oh constant in the second last equality not depend on $p$? – Lepidopterist Jan 2 '17 at 2:59
• I think the misunderstanding is that I didn't specify that p is the dimensionality of the matrices. – Lepidopterist Jan 2 '17 at 3:14

To make a very slight generalisation: I'll prove this for Hermitian, positive-semidefinite matrices.

Preliminaries:

From the inequality you give, and the fact that the spectra are contained in some compact interval $[\lambda_{min},\lambda_{max}]$, we can state:

$A_p$ and $B_p$ are positive-semidefinite Hermitian matrices, where $A_p$ has eigenvalues $\alpha_i$: $$\lambda_{max} \geq \alpha_1 \geq \alpha_2 \geq \cdots \geq \alpha_p \geq \lambda_{min} \geq 0,$$ and $B_p$ has eigenvalues $\beta_i$: $$\lambda_{max} \geq \beta_1 \geq \beta_2 \geq \cdots \geq \beta_p\geq \lambda_{min} \geq 0,$$ then by the Weyl matrix inequalities: $\forall i = 1,\dots,p$: $$|\alpha_i - \beta_i| \leq \|A_p-B_p\|_2 \leq 1/p.$$

Let us also note that the spectral norm $\|A_p\|_2$ of $A_p$ satisfies: $$\|A_p\|_2 = \max_{i = 1,\,\dots\,,\,p}|\alpha_i| = \alpha_1 \leq \lambda_{max}.$$

Finally, note that for $p \geq 2$ we have that $$\frac{1}{p-1} \leq \frac{2}{p}.$$

Claim:

For all $p = 1, 2, \dots$ , we have that $$\left\|\frac{p}{\operatorname{tr}(A_p)}A_p-B_p\right\|_2 \leq \frac{1+2\lambda_{max}}{p}$$

First of all, note that for $p=1$ the proof is quite trivial: $$\frac{p}{\operatorname{tr}(A_p)}A_p = \frac{A_p}{A_p} = 1$$ and $$B_p = \operatorname{tr}(B_p) = p = 1,$$ hence $$\left\|\frac{p}{\operatorname{tr}(A_p)}A_p-B_p\right\|_2 = \| 1 - 1 \|_2 = 0 \leq 1+2\lambda_{max}.$$

We will assume $p \geq 2$ from now on. Since $\operatorname{tr}(B_p) = p$ and $|\alpha_i - \beta_i| < 1/p$, we have that $$|\operatorname{tr}(A_p)-\operatorname{tr}(B_p)| = \left|\sum_{i=1}^p \alpha_i - \sum_{i=1}^p\beta_i\right|=\left|\sum_{i=1}^p (\alpha_i - \beta_i)\right| \leq \sum_{i=1}^p |\alpha_i - \beta_i| \leq \sum_{i=1}^p \frac{1}{p} = 1.$$ Define $\varepsilon = \operatorname{tr}(A_p) - \operatorname{tr}(B_p) \in [-1,1]$, such that $\operatorname{tr}(A_p) = \operatorname{tr}(B_p) + \varepsilon = p+\varepsilon > 0$.

Using our preliminaries, we get: \begin{align} \left\|\frac{p}{\operatorname{tr}(A_p)}A_p-B_p\right\|_2 &=\left\|\frac{p}{p+\varepsilon}A_p-B_p\right\|_2 \\ &=\left\|\frac{p + \varepsilon-\varepsilon}{p+\varepsilon}A_p-B_p\right\|_2 \\ &=\left\|A_p - B_p - \frac{\varepsilon}{p+\varepsilon}A_p\right\|_2 \\ &\leq \left\|A_p - B_p\right\|_2 + \left\|\frac{\varepsilon}{p+\varepsilon}A_p\right\|_2 \\ &\leq \frac{1}{p} + \frac{|\varepsilon|}{p+\varepsilon}\left\|A_p\right\|_2 \\ &\leq \frac{1}{p} + \frac{1}{p-1}\lambda_{max} \\ &\leq \frac{1}{p} + \frac{2}{p}\lambda_{max} \\ &= \frac{1+2\lambda_{max}}{p}. \end{align}