On varying coefficients of spatial inhomogeneous nonlinear Schrödinger equation

A nonlinear evolution equation for wave packet surface gravity waves with variation in topography is revisited in this article. The equation is modeled by a spatial inhomogeneous nonlinear Schrödinger (NLS) equation with varying coefficients, derived by Djordjević and Redekopp (1978) and the nonlinear coefficient is later corrected by Dingemans (1997). We show analytically and qualitatively that the nonlinear coefficient and the corresponding averaging value, stated but not derived, by Benilov, Flanagan and Howlin (2005) and Benilov and Howlin (2006) are inaccurate. For a particular choice of topography and wave characteristics, the NLS equation alternates between focusing and defocusing case and hence, it does not admit the formation of a classical soliton, neither bright nor dark one.