Entropy monotonic spectral method for Boltzmann equation

We propose a spectral method that discretizes the Boltzmann collision operator and satisfies a discrete version of the H-theorem. The method is obtained by modifying the existing Fourier spectral method to match a classical form of the discrete velocity method. It preserves the positivity of the solution on the Fourier collocation points and as a result satisfies the H-theorem. The fast algorithms appeared previously in the literature can be readily applied to this method to speed up the computation. A second-order convergence rate is validated by numerical experiments.