As a result, the ranges of the parameters were derived for stability of stellar systems of different geometries. Fridman discovered new types of non-Jeans instabilities in gravitating media, some of which develop at timescales less than Jeans' timescale, while others exist in systems that are stable according to Jeans. With V. Polyachenko, Fridman derived a new type of asinphase eigen oscillations that exist in multi-component gravitating media as non-collapsing waves with the wavelength above the critical (that would be impossible in a single-component media). In Fridman's works, it was first shown that solitons can exist in gravitating media, and transform into gravitating shock waves if dissipation is present. A. Fridman broke the traditional view that shock waves cannot exist in collisionless stellar systems, and proved the existence of "collisionless" shock waves in rotating stellar discs where the free path is the size of the epicycle. Fridman first showed that Landau nonlinear damping can exist in stellar systems. In the field of physics of planetary rings, Fridman with N. Gorkavyi developed the theory of transfer, collective and resonance processes in a system of non-elastic collisional gravitating particles, that explained the hierarchical structure of the Saturn rings, the resonance nature of the Uran rings, and allowed to predict the small satellites of Uran. The last theoretical prediction was then confirmed when 9 of the 10 satellites discovered by Voyager 2 were found in the predicted regions, with four on the pre-calculated orbits at less than 0.5% error. In collaboration with colleagues, Fridman developed the hydrodynamic theory of galactic spiral structure generation. The theory was proved in an original experiment on rotating shallow water that was engineered at Kurchatov Research Center. Using results of this experiment, also giant anticyclones in discs of spiral galaxies were predicted. The anticyclones were later observed, derived from maps of galactic velocities (data from the 6 meter telescope RAS. With O. Khoruzhii, Fridman developed nonlinear dynamics of astrophysics discs, deriving formation of mono and dipole vortices in discs, and the new type of accretion - acoustic drift - features that were later detected in planetary rings, and in the gas disc of our Galaxy. Fridman developed the theory of weak turbulence for rotating gravitating systems that explains the correlation between main parameters of gas clouds and their structure in our Galaxy, and the observed mass spectrum of the clouds. Seismodynamics and Ocean physics: with colleagues, Fridman discovered two