Let  $f: \mathbb{R}\to\mathbb{R}$ is a function such that $f( \cot x ) = \cos 2x+\sin 2x$ for all $0 < x < \pi$. Define $g(x) = f(x) f(1-x)$ for $-1 \leq x \leq 1$. Find the maximum and minimum values of $g$ on the closed interval $[-1, 1].$
1. **Express \( f(\cot x) \) in terms of trigonometric identities:**

   Given \( f(\cot x) = \cos 2x + \sin 2x \), we use the trigonometric identities:
   \[
   \cos 2x = \frac{\cot^2 x - 1}{\cot^2 x + 1} \quad \text{and} \quad \sin 2x = \frac{2 \cot x}{\cot^2 x + 1}
   \]
   Therefore,
   \[
   f(\cot x) = \cos 2x + \sin 2x = \frac{\cot^2 x - 1}{\cot^2 x + 1} + \frac{2 \cot x}{\cot^2 x + 1}
   \]
   Combining the fractions, we get:
   \[
   f(\cot x) = \frac{\cot^2 x - 1 + 2 \cot x}{\cot^2 x + 1} = \frac{\cot^2 x + 2 \cot x - 1}{\cot^2 x + 1}
   \]

2. **Generalize \( f(x) \) for all \( x \in \mathbb{R} \):**

   Since \( \cot x \) can take any real value as \( x \) varies over \( (0, \pi) \), we can write:
   \[
   f(x) = \frac{x^2 + 2x - 1}{x^2 + 1} \quad \forall x \in \mathbb{R}
   \]

3. **Define \( g(x) \) and simplify:**

   Given \( g(x) = f(x) f(1-x) \) for \( -1 \leq x \leq 1 \), we need to find \( f(1-x) \):
   \[
   f(1-x) = \frac{(1-x)^2 + 2(1-x) - 1}{(1-x)^2 + 1} = \frac{1 - 2x + x^2 + 2 - 2x - 1}{1 - 2x + x^2 + 1} = \frac{x^2 - 2x + 2}{x^2 - 2x + 2}
   \]
   Simplifying, we get:
   \[
   f(1-x) = \frac{x^2 - 2x + 2}{x^2 - 2x + 2} = 1
   \]
   Therefore,
   \[
   g(x) = f(x) \cdot 1 = f(x)
   \]

4. **Find the range of \( f(x) \) on \( [-1, 1] \):**

   We need to find the maximum and minimum values of \( f(x) \) on the interval \( [-1, 1] \):
   \[
   f(x) = \frac{x^2 + 2x - 1}{x^2 + 1}
   \]
   To find the critical points, we take the derivative and set it to zero:
   \[
   f'(x) = \frac{(2x + 2)(x^2 + 1) - (x^2 + 2x - 1)(2x)}{(x^2 + 1)^2} = \frac{2x^3 + 2x + 2x + 2 - 2x^3 - 4x^2 + 2x}{(x^2 + 1)^2} = \frac{6x}{(x^2 + 1)^2}
   \]
   Setting \( f'(x) = 0 \):
   \[
   6x = 0 \implies x = 0
   \]
   Evaluating \( f(x) \) at the endpoints and the critical point:
   \[
   f(-1) = \frac{(-1)^2 + 2(-1) - 1}{(-1)^2 + 1} = \frac{1 - 2 - 1}{1 + 1} = \frac{-2}{2} = -1
   \]
   \[
   f(0) = \frac{0^2 + 2(0) - 1}{0^2 + 1} = \frac{-1}{1} = -1
   \]
   \[
   f(1) = \frac{1^2 + 2(1) - 1}{1^2 + 1} = \frac{1 + 2 - 1}{1 + 1} = \frac{2}{2} = 1
   \]

5. **Determine the maximum and minimum values of \( g(x) \):**

   Since \( g(x) = f(x) \), the maximum and minimum values of \( g(x) \) on \( [-1, 1] \) are the same as those of \( f(x) \):
   \[
   \max g(x) = 1 \quad \text{and} \quad \min g(x) = -1
   \]

The final answer is \(\boxed{1 \text{ and } -1}\)