Meina Gao,Jing Zhang Small-divisor equation of higher order with large variable coefficient and application to the coupled KdV equation (372K, PDF) ABSTRACT. In this paper, we establish an estimate for the solutions of small-divisor equation of higher order with large variable coefficients. Then by formulating an infinite dimensional KAM theorem which allows for multiple normal frequencies and unbounded perturbations, we prove that there are many periodic solutions for the coupled KdV equation subject to small Hamiltonian perturbations.