Rolling simplexes and their commensurability. II (a lemma on the directrix and focus)

The law of central-square dynamics
$$ (x,y,z)''=-\frac{4\pi^2k}{(\alpha(x-a)+\beta(y-b)+\gamma(z-c)+\delta)^2}(x-a,y-b,z-c), $$
expressing the focusing of a plane wave at the point $(a,b,c)$ is discussed and justified.