Two sets of integers such that all elements of the sumset of the two sets are perfect squares

This paper is concerned with the problem of finding two sets of integers, $\{a_1,a_2,\dots$, $a_m\}$ and $\{b_1,b_2,\dots ,b_n\}$, such that all the $mn$ sums $a_i+b_j,i=1,\dots ,m,j=1,\dots ,n$, are perfect squares. A method is known for generating numerical examples of such sets when $m=2$ or 3 and $n$ is arbitrary. When both $m$ and $n$ exceed 2, only one two-parameter solution with $(m,n)=(4,4)$ has been published. In this paper we obtain several multi-parameter solutions of the problem in three cases when $(m,n)$ is $(3,3)$ or $(5,3)$ or $(4,4)$, and we indicate how more such solutions may be obtained.