Linear systems with a quadratic integral

Rubanovskii, 1974, On bifurcation and stability of steady motions in some problems of the dynamics of a rigid body, Prikl. Mat. Mekh., 38, 616 Williamson, 1936, On the algebraic problem concerning the normal forms of linear dynamical systems, Am. J. Math., 58, 141, 10.2307/2371062 Lyapunov, 1950 Berger, 1990, Vol. 2 Karapetyan, 1983, Stability of conservative and dissipative systems, 6, 3 Horn, 1985 Karapetyan, 1975, On the stability of equilibrium of non-holonomic systems, Prikl. Mat. Mekh., 39, 1135 Arnol'd, 1978