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True/False Question :

There exist an integer $r\geq 1$ and a symmetric matrix $A \in M_{r}\left ( \mathbb{R} \right )$ such that for all $n \in \mathbb{N}$, we have: $$2^{\sqrt{n}}\leq \left | tr\left ( A^{n} \right )\leq \right |2020 . 2^{\sqrt{n}}.$$
(Note : Mark $\emph{0}$ for False and $\emph{1}$ for True.)

1 Answer

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Since $A$ is a real symmetric matrix, it is diagonalizable over $\mathbb{R}$.

So there exist real eigenvalues $\lambda_1,\lambda_2,\dots,\lambda_r$

such that

$\operatorname{tr}(A^n)=\lambda_1^n+\lambda_2^n+\cdots+\lambda_r^n\qquad \text{for all } n\in\mathbb{N}.$

Let

$\rho=\max{1\le i\le r} |\lambda i|.$

We consider three cases.

$\textbf{Case 1: } \rho<1$

Then $\lambda_i^n\to 0$ for every $i$, hence $\operatorname{tr}(A^n)\to 0.$

But the condition requires $|\operatorname{tr}(A^n)|\ge 2^{\sqrt n},$ and $2^{\sqrt n}\to\infty$.

This is impossible.
 

$\textbf{Case 2:} \rho=1$.

Then $|\lambda_i^n|\le 1$ for every $i$, so $|\operatorname{tr}(A^n)| \le |\lambda1^n|+\cdots+|\lambda r^n| \le r.$

Thus $(|\operatorname{tr}(A^n)|)$ is bounded.

But again $2^{\sqrt n}\to\infty,$

so this is impossible.

$\textbf{Case 3: } \rho>1$.

Let the eigenvalues with absolute value $\rho$ be $\pm \rho.$

For even powers $n=2m$, we have $\lambda i^{2m}=|\lambda i|^{2m}.$

Hence every eigenvalue with $|\lambda_i|=\rho$ contributes exactly $\rho^{2m}$.

So if $k\ge 1$ is the number of eigenvalues satisfying $|\lambda_i|=\rho$, then

$\operatorname{tr}(A^{2m}) = k \rho^{2m} + \sum{|\lambda_i|}$

Therefore, $\operatorname{tr}(A^{2m}) \sim k\rho^{2m}\qquad (m\to\infty).$

So $|\operatorname{tr}(A^{2m})|$ grows exponentially like $\rho^{2m}$.

But the required upper bound is

$|\operatorname{tr}(A^{2m})| \le 2020\cdot 2^{\sqrt{2m}},$ and $2^{\sqrt{2m}}$ grows subexponentially.

An exponential function cannot be bounded above by a subexponential one for all large $m$.

This is impossible.

Since all three cases lead to contradictions, no such symmetric matrix $A$ can exist.

$\textbf{Conclusion:}$ The statement is $\boxed{\text{False}}.$
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