Geodesic
Rolling simplexes and their commensurability. II (a lemma on the directrix and focus)
Fundamentalʹnaâ i prikladnaâ matematika, Tome 19 (2014) no. 1, pp. 13-19 
O. V. Gerasimova. Rolling simplexes and their commensurability. II (a lemma on the directrix and focus). Fundamentalʹnaâ i prikladnaâ matematika, Tome 19 (2014) no. 1, pp. 13-19. http://geodesic.mathdoc.fr/item/FPM_2014_19_1_a1/
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Voir la notice de l'article provenant de la source Math-Net.Ru

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.

[1] Gerasimova O. V., “Rolling simplexes and their commensurability. I (aksioma i kriterii neszhimaemosti i lemma o momente)”, Fundament. i prikl. mat., 17:2 (2011/2012), 87–95

[2] Razmyslov Yu. P., “Raz'yasnenie k “Rolling simplexes and their commensurability” (uravneniya polya po Tikho Brage)”, Fundament. i prikl. mat., 17:4 (2011/2012), 193–215 | MR