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In a recent paper by Casado-Pascual [Phys. Rev. E \textbf{97}, 032219(2018)], directed motion of a sphere immersed in a viscous fluid and subjectedto zero-average biharmonic forces is studied. The author explains thedependence on the relative amplitude of the two harmonic components of theaverage terminal velocity from the perspective of a general formalism. In thisComment, this explanation is shown to be in general incorrect, while the theoryof ratchet universality together with the vibrational mechanics approachprovide a satisfactory explanation of major aspects of the observed phenomena.

Comment on "Directed motion of spheres by unbiased driving forces in viscous fluids beyond the Stokes' law regime"