Solve the system  $\begin{cases} x+ y +z = a \\
 x^2 + y^2 + z^2 = a^2 \\
 x^3 + y^3 +z^3 = a^3
\end{cases}$
To solve the given system of equations:
\[
\begin{cases}
x + y + z = a \\
x^2 + y^2 + z^2 = a^2 \\
x^3 + y^3 + z^3 = a^3
\end{cases}
\]
we will use the elementary symmetric polynomials \(e_1, e_2, e_3\) defined as follows:
\[
e_1 = x + y + z, \quad e_2 = xy + xz + yz, \quad e_3 = xyz
\]

1. From the first equation, we have:
\[
e_1 = x + y + z = a
\]

2. From the second equation, we use the identity for the sum of squares:
\[
x^2 + y^2 + z^2 = e_1^2 - 2e_2
\]
Given \(x^2 + y^2 + z^2 = a^2\), we substitute \(e_1\):
\[
a^2 = e_1^2 - 2e_2 \implies a^2 = a^2 - 2e_2 \implies 2e_2 = 0 \implies e_2 = 0
\]

3. From the third equation, we use the identity for the sum of cubes:
\[
x^3 + y^3 + z^3 = e_1^3 - 3e_1e_2 + 3e_3
\]
Given \(x^3 + y^3 + z^3 = a^3\), we substitute \(e_1\) and \(e_2\):
\[
a^3 = e_1^3 - 3e_1e_2 + 3e_3 \implies a^3 = a^3 - 0 + 3e_3 \implies 3e_3 = 0 \implies e_3 = 0
\]

4. Now, we have the polynomial whose roots are \(x, y, z\):
\[
u^3 - e_1u^2 + e_2u - e_3 = 0
\]
Substituting \(e_1 = a\), \(e_2 = 0\), and \(e_3 = 0\), we get:
\[
u^3 - au^2 = 0 \implies u^2(u - a) = 0
\]

5. The solutions to this polynomial are:
\[
u = 0 \quad \text{or} \quad u = a
\]

Thus, the possible values for \(x, y, z\) are \(0\) or \(a\). Therefore, the solutions for \((x, y, z)\) are:
\[
(x, y, z) \in \{(a, 0, 0), (0, a, 0), (0, 0, a)\}
\]

The final answer is \( \boxed{ (x, y, z) \in \{(a, 0, 0), (0, a, 0), (0, 0, a)\} } \)