For each positive integer $n$ we denote by $p(n)$ the greatest square less than or equal to $n$.
[list=a]
[*]Find all pairs of positive integers $(  m,n)$, with $m\leq n$, for which
\[ p(  2m+1) \cdot p(  2n+1)  =400 \]
[*]Determine the set $\mathcal{P}=\{  n\in\mathbb{N}^{\ast}\vert n\leq100\text{ and }\dfrac{p(n+1)}{p(n)}\notin\mathbb{N}^{\ast}\}$[/list]
### Part (a)
We need to find all pairs of positive integers \((m, n)\) with \(m \leq n\) such that:
\[ p(2m+1) \cdot p(2n+1) = 400 \]

1. **Identify the possible values of \(p(k)\):**
   The function \(p(k)\) returns the greatest square less than or equal to \(k\). Therefore, \(p(k) = \lfloor \sqrt{k} \rfloor^2\).

2. **List the possible values of \(p(2m+1)\) and \(p(2n+1)\):**
   We need to find pairs \((a, b)\) such that \(a \cdot b = 400\) and \(a\) and \(b\) are perfect squares. The possible pairs are:
   \[
   (1, 400), (4, 100), (16, 25), (20, 20)
   \]

3. **Determine the corresponding values of \(2m+1\) and \(2n+1\):**
   - For \(p(2m+1) = 1\), \(2m+1\) must be in the range \([1, 3]\). Thus, \(m = 0, 1\).
   - For \(p(2n+1) = 400\), \(2n+1\) must be in the range \([361, 400]\). Thus, \(n\) ranges from 180 to 199.
   - For \(p(2m+1) = 4\), \(2m+1\) must be in the range \([4, 8]\). Thus, \(m = 2, 3\).
   - For \(p(2n+1) = 100\), \(2n+1\) must be in the range \([81, 100]\). Thus, \(n\) ranges from 40 to 49.
   - For \(p(2m+1) = 16\), \(2m+1\) must be in the range \([16, 24]\). Thus, \(m = 8, 9, 10, 11\).
   - For \(p(2n+1) = 25\), \(2n+1\) must be in the range \([25, 35]\). Thus, \(n\) ranges from 12 to 17.
   - For \(p(2m+1) = 20\), \(2m+1\) must be in the range \([20, 24]\). Thus, \(m = 9, 10, 11\).
   - For \(p(2n+1) = 20\), \(2n+1\) must be in the range \([20, 24]\). Thus, \(n = 9, 10, 11\).

4. **Combine the ranges to find valid pairs \((m, n)\):**
   - For \((1, 400)\): No valid pairs since \(m\) and \(n\) ranges do not overlap.
   - For \((4, 100)\): Possible pairs are \((2, 40), (2, 41), \ldots, (2, 49), (3, 40), (3, 41), \ldots, (3, 49)\).
   - For \((16, 25)\): Possible pairs are \((8, 12), (8, 13), \ldots, (8, 17), (9, 12), (9, 13), \ldots, (9, 17), (10, 12), (10, 13), \ldots, (10, 17), (11, 12), (11, 13), \ldots, (11, 17)\).
   - For \((20, 20)\): Possible pairs are \((9, 9), (9, 10), (9, 11), (10, 10), (10, 11), (11, 11)\).

### Part (b)
We need to determine the set \(\mathcal{P} = \{ n \in \mathbb{N}^* \mid n \leq 100 \text{ and } \frac{p(n+1)}{p(n)} \notin \mathbb{N}^* \}\).

1. **Identify the values of \(p(n)\) and \(p(n+1)\):**
   - \(p(n)\) is the greatest square less than or equal to \(n\).
   - \(p(n+1)\) is the greatest square less than or equal to \(n+1\).

2. **Determine when \(\frac{p(n+1)}{p(n)}\) is not a natural number:**
   - If \(n\) is just before a perfect square, then \(p(n+1)\) will be the next perfect square, and \(\frac{p(n+1)}{p(n)}\) will not be a natural number.
   - For example, if \(n = k^2 - 1\), then \(p(n) = (k-1)^2\) and \(p(n+1) = k^2\).

3. **List the values of \(n\) up to 100:**
   - The perfect squares up to 100 are \(1, 4, 9, 16, 25, 36, 49, 64, 81, 100\).
   - The values of \(n\) just before these perfect squares are \(3, 8, 15, 24, 35, 48, 63, 80, 99\).

Thus, the set \(\mathcal{P}\) is:
\[
\mathcal{P} = \{3, 8, 15, 24, 35, 48, 63, 80, 99\}
\]

The final answer is \( \boxed{ (2, 40), (2, 41), \ldots, (2, 49), (3, 40), (3, 41), \ldots, (3, 49), (8, 12), (8, 13), \ldots, (8, 17), (9, 12), (9, 13), \ldots, (9, 17), (10, 12), (10, 13), \ldots, (10, 17), (11, 12), (11, 13), \ldots, (11, 17), (9, 9), (9, 10), (9, 11), (10, 10), (10, 11), (11, 11) } \) for part (a) and \(\{3, 8, 15, 24, 35, 48, 63, 80, 99\}\) for part (b).