Asymptotics of the number of 2-threshold functions

A $k$-threshold function on a rectangular grid of size $m \times n$ is the conjunction of $k$ threshold functions on the same domain. In this paper, we focus on the case $k=2$ and show that the number of two-dimensional 2-threshold functions is~$\dfrac{25}{12\pi^4} m^4 n^4 + o(m^4n^4)$.