In the microcanonical ensemble, one considers a system with fixed energy, volume, and number of particles. We take a system composed of formula_17 identical bosons, formula_18 of which have energy formula_6 and are distributed over formula_2 levels or states with the same energy formula_6, i.e. formula_2 is the degeneracy associated with energy formula_6 of total energy formula_24. Calculation of the number of arrangements of formula_18 particles distributed among formula_2 states is a problem of combinatorics. Since particles are indistinguishable in the quantum mechanical context here, the number of ways for arranging formula_18 particles in formula_2 boxes (for the formula_29th energy level) would be (see image on the right)