Suppose that $S\subseteq \mathbb Z$ has the following property: if $a,b\in S$, then $a+b\in S$. Further, we know that $S$ has at least one negative element and one positive element. Is the following statement true?
There exists an integer $d$ such that for every $x\in \mathbb Z$, $x\in S$ if and only if $d|x$.
[i]proposed by Mahyar Sefidgaran[/i]
1. **Define the sets \( N \) and \( P \):**
   We are given that \( S \subseteq \mathbb{Z} \) has the property that if \( a, b \in S \), then \( a + b \in S \). Additionally, \( S \) contains at least one negative element and one positive element. Define the sets:
   \[
   N = \{-x \mid x \in S, x < 0\}
   \]
   \[
   P = \{y \mid y \in S, y > 0\}
   \]
   Both \( N \) and \( P \) are non-empty.

2. **Find the minimum elements \( n \) and \( s \):**
   Since \( N \) and \( P \) are subsets of the natural numbers, which are well-ordered, they have minimum elements. Let:
   \[
   n = \min N
   \]
   \[
   s = \min P
   \]

3. **Show that \( n = s \):**
   Suppose \( n \neq s \). Without loss of generality, assume \( n < s \). Then \( s - n \in S \) because \( S \) is closed under addition. Since \( s \) and \( n \) are positive, \( s - n \) is also positive and less than \( s \), which contradicts the minimality of \( s \). Therefore, \( n = s \).

4. **Denote the common value \( d \):**
   Since \( n = s \), denote this common value by \( d \). Thus, \( d \in S \) and \( -d \in S \).

5. **Show that \( S \subseteq d\mathbb{Z} \):**
   For any \( y \in P \), we have \( y \geq d \). By the division algorithm, we can write:
   \[
   y = qd + r \quad \text{with} \quad 0 \leq r < d
   \]
   Since \( S \) is closed under addition and subtraction, \( r = y - qd \in S \). But \( r \) is a non-negative integer less than \( d \), and the only such \( r \) in \( S \) is \( 0 \) (by the minimality of \( d \)). Therefore, \( r = 0 \), implying \( y = qd \). A similar argument holds for negative elements in \( S \). Hence, \( S \subseteq d\mathbb{Z} \).

6. **Show that \( d\mathbb{Z} \subseteq S \):**
   Since \( d \in S \) and \( S \) is closed under addition, any integer multiple of \( d \) is in \( S \). Therefore, \( d\mathbb{Z} \subseteq S \).

7. **Conclude the equivalence:**
   Combining the two inclusions, we have \( S = d\mathbb{Z} \). Thus, there exists an integer \( d \) such that for every \( x \in \mathbb{Z} \), \( x \in S \) if and only if \( d \mid x \).

\(\blacksquare\)

The final answer is \( \boxed{ \text{There exists an integer } d \text{ such that for every } x \in \mathbb{Z}, x \in S \text{ if and only if } d \mid x. } \)