double layers подвiйнi шари псевдопотенцiал Сагдєєва 05C10 05C25 05C40 Original contributions Sagdeev potential function Sagdeev potential method Sagdeev pseudopotential concentrations of positrons and negative ions connectivity cyclic group dicyclic group dihedral group double-layer solutions fast-mode phase plane trajectories fast-mode solitary waves heavier negative ions isothermal positrons iзотермiчнi позитрони non-isothermal plasma phase velocity planarity power graph slow mode and fast mode phase velocity slow mode compressive solitons and double layers stream velocities temperatures of positive and negative ions two-temperature non-isothermal and isothermal electrons