When thermal equilibrium prevails at temperature T = T X = T Y, the rate of accumulation of energy vanishes so that q (ν, T X, T Y) = 0. It follows that in thermodynamic equilibrium, when T = T X = T Y, 0 = α α ν ν , X , Y ( T , T ) B ν ν ( T ) − − ϵ ϵ ν ν , X ( T ) B ν ν ( T ) .