Paper 3, Section II, B

Integrable Systems
Part II, 2009

Consider the partial differential equation

ut=unux+2k+1ux2k+1\frac{\partial u}{\partial t}=u^{n} \frac{\partial u}{\partial x}+\frac{\partial^{2 k+1} u}{\partial x^{2 k+1}}

where u=u(x,t)u=u(x, t) and k,nk, n are non-negative integers.

(i) Find a Lie point symmetry of ()(*) of the form

(x,t,u)(αx,βt,γu),(x, t, u) \longrightarrow(\alpha x, \beta t, \gamma u),

where (α,β,γ)(\alpha, \beta, \gamma) are non-zero constants, and find a vector field generating this symmetry. Find two more vector fields generating Lie point symmetries of (*) which are not of the form ()(* *) and verify that the three vector fields you have found form a Lie algebra.

(ii) Put ()(*) in a Hamiltonian form.