Say I have a gas consist of atoms or molecules. How do I find the number of atoms in that ensemble that have energy above/below a specific amount, say E? I mean, what is the function that I'll have to integrate from 0 to E or E to infinity (if that's what I have to do), to find the percentage of particles in the set that are above an energy level, in the case of microcanonical, canonical and grand canonical ensemble? Is it the Maxwell–Boltzmann distribution to energy? Or the partition function? Or the density of states as a function of energy?
Thank you :D