Consider the system of equations $\begin{cases} x^2 - 3y + p = z,  \\
y^2 - 3z + p = x, \\
z^2 - 3x + p = y \end{cases}$ with real parameter $p$.
a) For $p \ge 4$, solve the considered system in the field of real numbers.
b) Prove that for $p \in (1, 4)$ every real solution of the system satisfies $x = y = z$.

(Jaroslav Svrcek)
### Part (a)
1. Consider the given system of equations:
   \[
   \begin{cases}
   x^2 - 3y + p = z, \\
   y^2 - 3z + p = x, \\
   z^2 - 3x + p = y
   \end{cases}
   \]
2. Add the three equations:
   \[
   (x^2 - 3y + p) + (y^2 - 3z + p) + (z^2 - 3x + p) = z + x + y
   \]
3. Simplify the left-hand side:
   \[
   x^2 + y^2 + z^2 - 3(x + y + z) + 3p = x + y + z
   \]
4. Combine like terms:
   \[
   x^2 + y^2 + z^2 - 4(x + y + z) + 3p = 0
   \]
5. Rearrange the equation:
   \[
   x^2 + y^2 + z^2 - 4x - 4y - 4z + 3p = 0
   \]
6. Complete the square for each variable:
   \[
   (x-2)^2 + (y-2)^2 + (z-2)^2 + 3p - 12 = 0
   \]
7. Rearrange to isolate the squared terms:
   \[
   (x-2)^2 + (y-2)^2 + (z-2)^2 = 12 - 3p
   \]
8. For \( p \ge 4 \):
   - If \( p > 4 \), then \( 12 - 3p < 0 \), which implies no real solutions since the sum of squares cannot be negative.
   - If \( p = 4 \), then \( 12 - 3p = 0 \), which implies:
     \[
     (x-2)^2 + (y-2)^2 + (z-2)^2 = 0
     \]
     Therefore, \( x = 2 \), \( y = 2 \), and \( z = 2 \).

### Part (b)
1. We need to prove that for \( p \in (1, 4) \), every real solution of the system satisfies \( x = y = z \).
2. Assume \( x, y, z \ge 0 \). Suppose \( x < 0 \):
   - From the first equation: \( z = x^2 - 3y + p \), since \( x < 0 \), \( z \ge p - 3y \).
   - From the second equation: \( x = y^2 - 3z + p \), since \( x < 0 \), \( y \ge p - 3z \).
3. Since \( p > 1 \), we have:
   \[
   z^2 + p - y = 3x < 0 \quad \text{and} \quad y^2 - 3z + p = x < 0
   \]
   - This implies \( y > z^2 + p \) and \( 3z > y^2 + p \).
4. Since \( y > z^2 + p \) and \( 3z > y^2 + p \), we get \( y, z > 0 \).
5. From \( p > 1 \), note that:
   \[
   z^2 - y + p < 0 \le (z - \sqrt{p})^2 \implies y > 2z \sqrt{p} > 2z
   \]
   \[
   y^2 - 3z + p < 0 \le (y - \sqrt{p})^2 \implies z > \frac{2}{3} y \sqrt{p} > \frac{2}{3} y
   \]
6. This leads to a contradiction since \( y > 2z > \frac{4}{3} y \).
7. Therefore, \( x, y, z \ge 0 \).
8. Assume \( \max\{x, y, z\} = x \):
   - From the first equation: \( z = x^2 - 3y + p \ge y \).
   - From the second equation: \( y = y^2 - 3z + p \ge x \).
9. Thus, \( x \ge z \ge y \) and \( z = x \).
10. Substituting \( z = x \) into the first equation:
    \[
    x^2 - 3y + p = x
    \]
11. Substituting \( z = x \) into the second equation:
    \[
    y^2 - 3x + p = x
    \]
12. Subtracting the two equations:
    \[
    3(x - y) = x - y \implies x = y
    \]
13. Therefore, \( x = y = z \).

The final answer is \( \boxed{ x = y = z = 2 } \) for \( p = 4 \) and \( x = y = z \) for \( p \in (1, 4) \).