Propagation of large amplitude ion-acoustic solitary waves (IASWs) in a fullyrelativistic plasma consisting of cold ions and ultrarelativistic hot electronsand positrons is investigated using the Sagdeev's pseudopotential method in arelativistic hydrodynamics model. Effects of streaming speed of plasma fluid,thermal energy, positron density and positron temperature on large amplitudeIASWs are studied by analysis of the pseudopotential structure. It is foundthat in regions that the streaming speed of plasma fluid is larger than that ofsolitary wave, by increasing the streaming speed of plasma fluid the depth andwidth of potential well increases and resulting in narrower solitons withlarger amplitude. This behavior is opposite for the case where the streamingspeed of plasma fluid is smaller than that of solitary wave. On the other hand,increase of the thermal energy results in wider solitons with smalleramplitude, because the depth and width of potential well decreases in thatcase. Additionally, the maximum soliton amplitude increases and the widthbecomes narrower as a result of increase in positron density. It is shown thatvarying the positron temperature does not have considerable effect on the widthand amplitude of IASWs. The existence of stationary soliton-like arbitaryamplitude waves is also predicted in fully relativistic electron-positron-ion(EPI) plasmas. Effects of streaming speed of plasma fluid, thermal energy,positron density and positron temperature on these kinds of solitons are thesame as that for large amplitude IASWs.
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