Comment on "Asymptotic Phase for Stochastic Oscillators"

Definition of the phase of oscillations is straightforward for deterministic periodic processes but nontrivial for stochastic ones. Recently, Thomas and Lindner in [Phys. Rev. Lett., v. 113, 254101 (2014)] suggested to use the argument of the complex eigenfunction of the backward density evolution operator with the smallest real part of the eigenvalue, as an asymptotic phase of stochastic oscillations. Here I show that this definition does not generally provide a correct asymptotic phase.