13. Given a positive integer $n \geqslant 5$, try to find:
(1) In the set of $n$ elements $\left\{a_{1}, a_{2}, \cdots, a_{n}\right\}$, how many different numbers are generated by $a_{i}+a_{j}(1 \leqslant i<j \leqslant n)$ at least;
(2) Determine all $n$-element sets that achieve the above minimum value.
13. (1) $2 n-3$. (2) $a_{1}, a_{2}, \cdots \cdots, a_{n}$ form an arithmetic sequence.