Determine if there exists a subset $E$ of $Z \times Z$ with the properties:
(i) $E$ is closed under addition,
(ii) $E$ contains $(0,0),$
(iii) For every $(a,b) \ne (0,0), E$ contains exactly one of $(a,b)$ and $-(a,b)$.

Remark: We define $(a,b)+(a',b') = (a+a',b+b')$ and $-(a,b) = (-a,-b)$.
To determine if there exists a subset \( E \subseteq \mathbb{Z} \times \mathbb{Z} \) with the given properties, we need to verify the following conditions:
1. \( E \) is closed under addition.
2. \( E \) contains \((0,0)\).
3. For every \((a,b) \ne (0,0)\), \( E \) contains exactly one of \((a,b)\) and \(- (a,b)\).

Let's consider the subset \( E = (\mathbb{Z}_{>0} \times \mathbb{Z}) \cup (\{0\} \times \mathbb{Z}_{\ge 0}) \).

1. **Check if \( E \) is closed under addition:**

   - If \((a,b) \in \mathbb{Z}_{>0} \times \mathbb{Z}\) and \((c,d) \in \mathbb{Z}_{>0} \times \mathbb{Z}\), then \((a,b) + (c,d) = (a+c, b+d)\). Since \(a > 0\) and \(c > 0\), \(a+c > 0\), and thus \((a+c, b+d) \in \mathbb{Z}_{>0} \times \mathbb{Z}\).
   - If \((a,b) \in \{0\} \times \mathbb{Z}_{\ge 0}\) and \((c,d) \in \{0\} \times \mathbb{Z}_{\ge 0}\), then \((a,b) + (c,d) = (0, b+d)\). Since \(b \ge 0\) and \(d \ge 0\), \(b+d \ge 0\), and thus \((0, b+d) \in \{0\} \times \mathbb{Z}_{\ge 0}\).
   - If \((a,b) \in \mathbb{Z}_{>0} \times \mathbb{Z}\) and \((c,d) \in \{0\} \times \mathbb{Z}_{\ge 0}\), then \((a,b) + (c,d) = (a, b+d)\). Since \(a > 0\) and \(d \ge 0\), \((a, b+d) \in \mathbb{Z}_{>0} \times \mathbb{Z}\).

   Therefore, \( E \) is closed under addition.

2. **Check if \( E \) contains \((0,0)\):**

   - Clearly, \((0,0) \in \{0\} \times \mathbb{Z}_{\ge 0}\), so \( E \) contains \((0,0)\).

3. **Check if \( E \) contains exactly one of \((a,b)\) and \(- (a,b)\) for every \((a,b) \ne (0,0)\):**

   - For \((a,b) \in \mathbb{Z}_{>0} \times \mathbb{Z}\), \((a,b) \in E\) and \(- (a,b) = (-a, -b) \notin E\) because \(-a \le 0\).
   - For \((0,b) \in \{0\} \times \mathbb{Z}_{\ge 0}\) with \(b > 0\), \((0,b) \in E\) and \(- (0,b) = (0, -b) \notin E\) because \(-b < 0\).
   - For \((0,b) \in \{0\} \times \mathbb{Z}_{\ge 0}\) with \(b = 0\), \((0,0) \in E\) and \(- (0,0) = (0,0) \in E\), which is consistent with the condition since \((0,0)\) is the only element that can be in \( E \) and its negative.

Thus, the subset \( E = (\mathbb{Z}_{>0} \times \mathbb{Z}) \cup (\{0\} \times \mathbb{Z}_{\ge 0}) \) satisfies all the given properties.

The final answer is \( \boxed{ E = (\mathbb{Z}_{>0} \times \mathbb{Z}) \cup (\{0\} \times \mathbb{Z}_{\ge 0}) } \)