On the Classification of Fifth Order Quasi-linear Non-constant Separant Scalar Evolution Equations of the KdV-type

Abstract

Fifth order, quasi-linear, non-constant separant evolution equations are of the form u_t=A\frac{\partial^5 u}{\partial x^5}+\tilde{B}, where A and \tilde{B} are functions of x, t, u and of the derivatives of u with respect to x up to order 4. We use the existence of a "formal symmetry", hence the existence of "canonical conservation laws" \rho_{(i)}, i=-1,...,5 as an integrability test. We define an evolution equation to be of the KdV-Type, if all odd numbered canonical conserved densities are nontrivial. We prove that fifth order, quasi-linear, non-constant separant evolution equations of KdV type are polynomial in the function a=A^{1/5}; a=(\alpha u_3^2 +\beta u_3+\gamma)^{-1/2}, where \alpha, \beta and \gamma are functions of x, t, u and of the derivatives of u with respect to x up to order 2. We determine the u_2 dependency of a in terms of P=4\alpha\gamma-\beta^2>0 and we give an explicit solution, showing that there are integrable fifth order non-polynomial evolution equations.