For example, the population numbers for two particles in three sublevels are 200, 110, 101, 020, 011, or 002 for a total of six which equals 4!/(2!2!). The number of ways that a set of occupation numbers n i can be realized is the product of the ways that each individual energy level can be populated: W = ∏ ∏ i w ( n i , g i ) = ∏ ∏ i ( n i + g i − − 1 ) ! n i ! ( g i − − 1 ) ! ≈ ≈ ∏ ∏ i ( n i + g i ) ! n i ! ( g i ) ! where the approximation assumes that n i ≫ ≫ 1 .