The rotational kinetic energy of a rotator with three degrees of freedom can be written as
εrot=2I1Mξ2+2I2Mη2+2I3Mζ2,
where (ξ,η,ζ) are coordinates in a rotating frame of reference whose axes coincide with the principal axes of the rotator, while (Mξ,Mη,Mζ) are the corresponding angular momenta. Carrying out integrations in the phase space of the rotator, derive expression (6.5.41) for the partition function jrot(T) in the classical approximation.