1. A. P. Pyatakov and A. K. Zvezdin, “Magnetoelectric and multiferroic media,” Phys. Usp. 55 (6), 557–581 (2012).
2. C. Lanczos, The Variational Principles of Mechanics (University of Toronto Press, Toronto, 1962).
3. É. M. Galeev and V. M. Tikhomirov, Optimization: Theory, Examples, Problems (Editorial URSS, Moscow, 2000) [in Russian].
4. L. A. Kalyakin, A. K. Zvezdin, and Z. V. Gareeva, “Asymptotic analysis of a multiferroic model,” Theoret. and Math. Phys. 203 (1), 457–468 (2020).
5. Z. V. Gareeva, L. A. Kalyakin, I. R. Kayumov, and A. K. Zvezdin, “Spin-reorientation transitions in multiferroics with cycloidal spin ordering,” Physics of Metals and Metallography (PMM) 121 (4), 310–315 (2020).