Let $1<k\leq n$ be positive integers and $x_1 , x_2 , \ldots , x_k$ be positive real numbers such that $x_1 \cdot x_2 \cdot \ldots \cdot x_k  = x_1 + x_2 + \ldots +x_k.$

a) Show that $x_{1}^{n-1} +x_{2}^{n-1} + \ldots +x_{k}^{n-1} \geq kn.$

b) Find all numbers $k,n$ and $x_1, x_2 ,\ldots , x_k$ for which equality holds.
1. **Given Conditions and Initial Setup:**
   We are given that \(1 < k \leq n\) are positive integers and \(x_1, x_2, \ldots, x_k\) are positive real numbers such that:
   \[
   x_1 \cdot x_2 \cdot \ldots \cdot x_k = x_1 + x_2 + \ldots + x_k.
   \]
   We need to show that:
   \[
   x_1^{n-1} + x_2^{n-1} + \ldots + x_k^{n-1} \geq kn.
   \]

2. **Applying AM-GM Inequality:**
   By the Arithmetic Mean-Geometric Mean (AM-GM) inequality, we have:
   \[
   \frac{x_1 + x_2 + \ldots + x_k}{k} \geq \sqrt[k]{x_1 \cdot x_2 \cdot \ldots \cdot x_k}.
   \]
   Given \(x_1 \cdot x_2 \cdot \ldots \cdot x_k = x_1 + x_2 + \ldots + x_k\), we can rewrite the AM-GM inequality as:
   \[
   \frac{P}{k} \geq \sqrt[k]{P},
   \]
   where \(P = x_1 \cdot x_2 \cdot \ldots \cdot x_k\).

3. **Simplifying the Inequality:**
   From the above inequality, we get:
   \[
   P \geq k \sqrt[k]{P}.
   \]
   Let \(P = k^k\). Then:
   \[
   k^k \geq k \cdot k^{\frac{k}{k}} = k \cdot k = k^2.
   \]
   This implies:
   \[
   P \geq k^k.
   \]

4. **Using Bernoulli's Inequality:**
   We now use Bernoulli's inequality, which states that for any real number \(r \geq 0\) and integer \(x \geq 0\):
   \[
   (1 + x)^r \geq 1 + rx.
   \]
   Applying this to our problem, we get:
   \[
   k^r = (1 + (k-1))^r \geq 1 + r(k-1).
   \]

5. **Combining Results:**
   We now combine the results to show:
   \[
   \sum_{i=1}^k x_i^{n-1} \geq kP^{\frac{n-1}{k}} \geq k \cdot k^{\frac{n-1}{k-1}} \geq k \left(1 + (k-1) \frac{n-1}{k-1}\right) = kn.
   \]

6. **Equality Case:**
   For equality to hold, we need \(x_1 = x_2 = \ldots = x_k\). Let \(x_1 = x_2 = \ldots = x_k = x\). Then:
   \[
   x^k = kx \implies x^{k-1} = k \implies x = k^{\frac{1}{k-1}}.
   \]
   Therefore, the equality holds when \(x_1 = x_2 = \ldots = x_k = k^{\frac{1}{k-1}}\).

The final answer is \( \boxed{ x_1^{n-1} + x_2^{n-1} + \ldots + x_k^{n-1} \geq kn } \).