To obtain λ λ 1 , one substitutes the probability into the first constraint: 1 = ∑ ∑ i ρ ρ i = exp ⁡ ⁡ ( − − k B + λ λ 1 k B ) Z , where Z is a constant number defined as the canonical ensemble partition function : Z ≡ ≡ ∑ ∑ i exp ⁡ ⁡ ( λ λ 2 k B E i ) .