On the extension of $D(-8k^2)$-pair $\{8k^2, 8k^2+1\}$

Abstract: Let $n$ be a nonzero integer. A set of $m$ positive integers is called a $D(n)$-$m$-tuple if the product of any two of its distinct elements increased by $n$ is a perfect square. Let $k$ be a positive integer. By elementary means, we show that the $D(-8k^2)$-pair ${8k^2, 8k^2+1}$ can be extended to at most a quadruple (the third and fourth element can only be $1$ and $32k^2+1$). At the end, we suggest considering a $D(-k^2)$-triple ${ 1, 2k^2, 2k^2+2k+1}$ as possible future research direction.