[A.M. Kamchatnov "Topological solitons in magnetohydrodynamics", Sov. Phvs. JETP 55, 69 (1982) ref, arXiv:physics/0409093v1]. The author has found exact solution of the equations of magnetohydrodynamics which describes soliton with Hopf index = 1, i.e. simpliest hopfion.
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