1. I. M. Krichever, “Algebraic-geometric $$n$$-orthogonal curvilinear coordinate systems and the solution of associativity equations”, Funkts. Anal. Prilozhen., 31:1 (1997), 32–50; English transl.: Functional Anal. Appl., 31:1 (1997), 25–39.
2. S. P. Tsarev, “The geometry of hamiltonian systems of hydrodynamic type. The generalized hodograph method”, Izv. Akad. Nauk SSSR, Ser. Mat., 54:5 (1990), 1048–1068; English transl.: Math. USSR-Izv., 37:2 (1991), 397–419.
3. G. Darboux, Leçons sur les systèmes orthogonaux et les coordonnées curvilignes, Gauthier-Villars, Paris, 1910.
4. B. A. Dubrovin and S. P. Novikov, “Hamiltonian formalism of one-dimensional systems of the hydrodynamic type, and the Bogolyubov–Whitham averaging method”, Dokl. Akad. Nauk SSSR, 270:4 (1983), 781–785; English transl.: Soviet Math. Dokl., 27 (1983), 665–669.
5. O. I. Mokhov and E. V. Ferapontov, “Nonlocal Hamiltonian operators of hydrodynamic type that are connected with metrics of constant curvature”, Uspekhi Mat. Nauk, 45:3(273) (1990), 191–192; English transl.: Russian Math. Surveys, 45:3 (1990), 218–219.