The set $M= \{1;2;3;\ldots ; 29;30\}$ is  divided in $k$
subsets such that if $a+b=n^2, (a,b \in M, a\neq b, n$ is an
integer number $)$, then $a$ and $b$ belong different subsets.
Determine the minimum value of $k$.
1. **Identify the pairs \((a, b)\) such that \(a + b = n^2\):**
   We need to find all pairs \((a, b)\) in the set \(M = \{1, 2, 3, \ldots, 30\}\) such that \(a + b = n^2\) for some integer \(n\). The possible values of \(n^2\) within the range of \(M\) are \(4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900\).

2. **List the pairs \((a, b)\) for each \(n^2\):**
   - For \(n^2 = 4\): \((1, 3)\)
   - For \(n^2 = 9\): \((2, 7), (3, 6), (4, 5)\)
   - For \(n^2 = 16\): \((1, 15), (2, 14), (3, 13), (4, 12), (5, 11), (6, 10), (7, 9)\)
   - For \(n^2 = 25\): \((1, 24), (2, 23), (3, 22), (4, 21), (5, 20), (6, 19), (7, 18), (8, 17), (9, 16), (10, 15), (11, 14), (12, 13)\)
   - For \(n^2 = 36\): \((6, 30), (7, 29), (8, 28), (9, 27), (10, 26), (11, 25), (12, 24), (13, 23), (14, 22), (15, 21), (16, 20), (17, 19)\)

3. **Determine the minimum value of \(k\):**
   - We need to ensure that no two elements \(a\) and \(b\) such that \(a + b = n^2\) are in the same subset.
   - For \(k = 2\), we can try to divide the set \(M\) into two subsets. However, it is not possible to do so without violating the condition. For example, if we place \(1\) in one subset, then \(3\) must be in the other subset. But then, for \(n^2 = 16\), \(1\) and \(15\) must be in different subsets, which is not possible with only two subsets.
   - Therefore, \(k = 2\) is not sufficient.

4. **Example for \(k = 3\):**
   - We can divide the set \(M\) into three subsets such that the condition is satisfied. For example:
     - Subset 1: \(\{1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22, 23, 25, 26, 28, 29\}\)
     - Subset 2: \(\{3, 6, 9, 12, 15, 18, 21, 24, 27, 30\}\)
     - Subset 3: \(\{1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22, 23, 25, 26, 28, 29\}\)
   - This division ensures that no two elements \(a\) and \(b\) such that \(a + b = n^2\) are in the same subset.

The final answer is \(\boxed{3}\).