In the microcanonical ensemble, one considers a system with fixed energy, volume, and number of particles. We take a system composed of N = ∑ ∑ i n i identical bosons, n i of which have energy ε ε i and are distributed over g i levels or states with the same energy ε ε i , i.e. g i is the degeneracy associated with energy ε ε i of total energy E = ∑ ∑ i n i ε ε i . Calculation of the number of arrangements of n i particles distributed among g i states is a problem of combinatorics. Since particles are indistinguishable in the quantum mechanical context here, the number of ways for arranging n i particles in g i boxes (for the i th energy level) would be (see image):