Find all real values of a for which the equation $ (a \minus{} 3x^2 \plus{} \cos \frac {9\pi x}{2})\sqrt {3 \minus{} ax} \equal{} 0$ has an odd number of solutions in the interval $ [ \minus{} 1,5]$
To find all real values of \( a \) for which the equation \( (a - 3x^2 + \cos \frac{9\pi x}{2})\sqrt{3 - ax} = 0 \) has an odd number of solutions in the interval \([ -1, 5 ]\), we need to analyze the equation step by step.

1. **Analyze the equation**:
   \[
   (a - 3x^2 + \cos \frac{9\pi x}{2})\sqrt{3 - ax} = 0
   \]
   For the product to be zero, either \( (a - 3x^2 + \cos \frac{9\pi x}{2}) = 0 \) or \( \sqrt{3 - ax} = 0 \).

2. **Case 1: \( \sqrt{3 - ax} = 0 \)**:
   \[
   \sqrt{3 - ax} = 0 \implies 3 - ax = 0 \implies x = \frac{3}{a}
   \]
   For \( x \) to be in the interval \([-1, 5]\):
   \[
   -1 \leq \frac{3}{a} \leq 5
   \]
   This inequality can be split into two parts:
   \[
   -1 \leq \frac{3}{a} \quad \text{and} \quad \frac{3}{a} \leq 5
   \]

   - For \( -1 \leq \frac{3}{a} \):
     \[
     -1 \leq \frac{3}{a} \implies a \leq -3 \quad \text{or} \quad a \geq 3
     \]

   - For \( \frac{3}{a} \leq 5 \):
     \[
     \frac{3}{a} \leq 5 \implies a \geq \frac{3}{5} \quad \text{or} \quad a \leq -\frac{3}{5}
     \]

   Combining these conditions:
   \[
   a \leq -3 \quad \text{or} \quad a \geq 3
   \]

3. **Case 2: \( a - 3x^2 + \cos \frac{9\pi x}{2} = 0 \)**:
   \[
   a - 3x^2 + \cos \frac{9\pi x}{2} = 0
   \]
   This equation must be analyzed for the number of solutions in the interval \([-1, 5]\).

   - The function \( \cos \frac{9\pi x}{2} \) is periodic with period \(\frac{4}{9}\).
   - The function \( 3x^2 \) is a parabola opening upwards.

   We need to find the values of \( a \) such that the equation \( a - 3x^2 + \cos \frac{9\pi x}{2} = 0 \) has an odd number of solutions.

4. **Combining both cases**:
   - If \( a \leq -3 \) or \( a \geq 3 \), the equation \( \sqrt{3 - ax} = 0 \) gives one solution \( x = \frac{3}{a} \).
   - For \( a \in [-3, \frac{3}{5}] \), we need to check the number of solutions of \( a - 3x^2 + \cos \frac{9\pi x}{2} = 0 \).

   Since \( 3x^2 \) is an even function and \( \cos \frac{9\pi x}{2} \) is periodic, the number of solutions of \( a - 3x^2 + \cos \frac{9\pi x}{2} = 0 \) is even. Therefore, the total number of solutions is odd if and only if there is one solution from \( \sqrt{3 - ax} = 0 \).

Hence, the values of \( a \) for which the equation has an odd number of solutions in the interval \([-1, 5]\) are:
\[
a \leq -3 \quad \text{or} \quad a \geq 3
\]

The final answer is \( \boxed{ a \leq -3 } \) or \( a \geq 3 \).