SkrypnikW. I. “Long-Range Order in Linear Ferromagnetic Oscillator Systems. Strong Pair Quadratic N-N Potential”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 6, June 2004, pp. 810–817, https://umj.imath.kiev.ua/index.php/umj/article/view/3800.