Question 3
Let $n>3$ be an integer and let $T=\{1,2,\ldots,n\}$. A subset $S$ of $T$ is called deficient in $T$ if there is a positive integer $c\le n/2$ such that $|s_1-s_2|\ne c$ for any two elements $s_1,s_2$ of $S$. What is the largest possible number of elements of a deficient subset?