<s>[INST] Find the eigenvalues of a symmetric matrix 
Find the eigenvalues of a $3 \times 3$ symmetric matrix with $1$ on the main diagonal and  $\frac{1}{\sqrt 3}$ off the main diagonal.

Since each row on addition give the same value, one of the three eigenvalue is $1+\frac{2}{\sqrt 3}$.
Is there an easy way to find the other two values without using the formula $\det(A-\lambda I_3) = 0$.
 [/INST] You might want to take advantage that your $3 \times 3 $ matrix is a circulant matrix of the form:
$$ \begin{bmatrix} c_0 & c_{n-1} & c_{n-2} & \cdots & c_1 \\ c_1 & c_0 & c_{n-1} & \cdots & c_{2}\\ \vdots & c_1& c_0& \ddots & \vdots \\ c_{n-2} & &\ddots & \ddots& c_{n-1}\\ c_{n-1} & c_{n-2} &\cdots & c_1 & c_{0}  \end{bmatrix}$$ and the eigenvalues can be computed through the formula:
$$ \lambda_j = c_0 + c_1 \omega_j +c_2 \omega_j^2 + \cdots+ c_{n-1}\omega_j^{n-1}, \quad j = 0,1,\ldots,n-1,\tag 1$$
where $\omega_j = \exp\left(\frac{2 \pi i j}{n}\right)$ are the $n-$th roots of unity.
Concretely, $n = 3$ and we have $\omega_0 = 1, \omega_1 = -\frac{1}{2} + \frac{i\sqrt 3}{2}, \omega_2 = -\frac{1}{2} - \frac{i\sqrt 3}{2}.$
According to $(1)$:
$$\lambda_0 = 1 + \frac{1}{\sqrt 3} \cdot 1^1 + \frac{1}{\sqrt 3}\cdot 1^2 = 1 +\frac{2}{\sqrt 3}. $$
$$\lambda_1 = 1 + \frac{1}{\sqrt 3}\cdot \left(-\frac 12 +\frac{i\sqrt 3}{2}\right) + \frac{1}{\sqrt 3}\cdot \left(-\frac 12 +\frac{i\sqrt 3}{2}\right)^2   = 1-\frac{1}{\sqrt 3}.$$
Similarly, $\lambda_2 =  1 - \frac{1}{\sqrt 3}.$
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