I have become almost sure but have only some small doubts. Are all isothermal process actually ideal gas equation PV=mRT? If all such processes are occur in closed systems, this is so. Because it is isothermal the temperature is constant, R is constant and so is mass for a closed system. So the left side of the equation should be PV, which is a polytropic process with n=1. But my doubts comes from that in books PV=mRT is always called ideal gas equation, not a polytropic process with n=1 and when it comes to a isothermal process they use ideal gas equation.