9 Given the set of positive real numbers $A=\left\{a_{1}, a_{2}, a_{3}, \cdots, a_{100}\right\}$, let the set $S=$ $\{(a, b) \mid a \in A, b \in A, a-b \in A\}$. Then the maximum number of elements in set $S$ is.
9 4950 Hint: The number of ordered pairs of real numbers $(a, b)$ formed by elements in $A$ is $100^{2}=10000$ (pairs), because $a_{i}-a_{i}=0 \notin A$, hence $\left(a_{i}, a_{i}\right) \notin S(i=1$, $2, \cdots, 100)$; when $\left(a_{i}, a_{j}\right) \in S$, $\left(a_{j}, a_{i}\right) \notin S$, thus the number of elements in the set $S$ is at most $\frac{1}{2}(10000-100)=4950$. When $a_{i}=i(i=1,2, \cdots, 100)$, the number of elements in $S$ is 4950.