 Let $(G, \cdot)$  a finite group with order $n \in \mathbb{N}^{*},$ where $n \geq 2.$ We will say that group $(G, \cdot)$ is arrangeable if there is an ordering of its elements, such that 

    \[
    G = \{ a_1, a_2, \ldots, a_k, \ldots , a_n \} = \{ a_1 \cdot a_2, a_2 \cdot a_3, \ldots, a_k \cdot a_{k + 1}, \ldots , a_{n} \cdot a_1 \}.
    \]

    a) Determine all positive integers $n$ for which the group $(Z_n, +)$ is arrangeable.

    b) Give an example of a group of even order that is arrangeable.
### Part (a)

1. **Understanding the Problem:**
   We need to determine all positive integers \( n \) for which the group \((\mathbb{Z}_n, +)\) is arrangeable. The group \((\mathbb{Z}_n, +)\) consists of integers modulo \( n \) under addition.

2. **Sum of Elements in \(\mathbb{Z}_n\):**
   The elements of \(\mathbb{Z}_n\) are \(\{0, 1, 2, \ldots, n-1\}\). The sum of all elements in \(\mathbb{Z}_n\) is:
   \[
   \sum_{k=0}^{n-1} k = \frac{n(n-1)}{2}
   \]

3. **Condition for Arrangeability:**
   For \((\mathbb{Z}_n, +)\) to be arrangeable, there must exist an ordering of its elements such that:
   \[
   \{0, 1, 2, \ldots, n-1\} = \{a_1 + a_2, a_2 + a_3, \ldots, a_{n-1} + a_n, a_n + a_1\} \pmod{n}
   \]
   This implies that the sum of all elements in the set \(\{a_1 + a_2, a_2 + a_3, \ldots, a_{n-1} + a_n, a_n + a_1\}\) must be congruent to the sum of all elements in \(\{0, 1, 2, \ldots, n-1\}\) modulo \( n \).

4. **Sum of Pairwise Sums:**
   The sum of the pairwise sums is:
   \[
   (a_1 + a_2) + (a_2 + a_3) + \cdots + (a_{n-1} + a_n) + (a_n + a_1)
   \]
   Each element \(a_i\) appears exactly twice in this sum, so:
   \[
   2(a_1 + a_2 + \cdots + a_n) = 2 \sum_{k=0}^{n-1} k = 2 \cdot \frac{n(n-1)}{2} = n(n-1)
   \]
   Since \(n(n-1)\) is always divisible by \(n\), the sum of the pairwise sums modulo \(n\) is \(0\).

5. **Odd \(n\) Suffices:**
   For \(n\) odd, we can take \(a_k = k\) for all \(k\). This means:
   \[
   \{0, 1, 2, \ldots, n-1\} = \{1, 2, 3, \ldots, n-1, 0\}
   \]
   which is a cyclic permutation of the original set, thus satisfying the condition.

6. **Conclusion:**
   Therefore, all positive integers \(n\) for which \((\mathbb{Z}_n, +)\) is arrangeable are the odd integers.

### Part (b)

1. **Example of an Arrangeable Group of Even Order:**
   Consider the group \(D_4\) (the dihedral group of order 8). The elements of \(D_4\) can be written as \(\{e, r, r^2, r^3, s, sr, sr^2, sr^3\}\), where \(r\) is a rotation and \(s\) is a reflection.

2. **Arrangeability of \(D_4\):**
   We can arrange the elements of \(D_4\) as follows:
   \[
   \{e, r, r^2, r^3, s, sr, sr^2, sr^3\}
   \]
   and consider the products:
   \[
   \{e \cdot r, r \cdot r^2, r^2 \cdot r^3, r^3 \cdot s, s \cdot sr, sr \cdot sr^2, sr^2 \cdot sr^3, sr^3 \cdot e\}
   \]
   This set is a permutation of the original set, thus \(D_4\) is arrangeable.

The final answer is \( \boxed{ n } \) is odd for part (a) and \(D_4\) for part (b).