  A procedure to solve few-body problems which is based on an expansion over a
small parameter is developed. The parameter is the ratio of potential energy to
kinetic energy in the subspace of states having not small hyperspherical
quantum numbers, K>K_0. Dynamic equations are reduced perturbatively to those
in the finite subspace with K \le K_0. The contribution from the subspace with
K>K_0 is taken into account in a closed form, i.e. without an expansion over
basis functions.
