Can anyone help me with this problem?
We call a set $A\subseteq \mathbb R^n$ a $2$-distance set if for each $v_i,v_j$ in $A$, $i\neq j$, $|v_i-v_j|=r$ or $s$. Find an upper bound for the number of the elements of $A$.
Our Teacher proved that $|A|\le \frac{n^2+5n+4}{2}$ using linear algebra, but I read somewhere that $|A|\le \frac{n^2+3n+4}{2}$. Any good upper bound is welcomed!