  The U(1) Calogero-Sutherland Model with anti-periodic boundary condition is
studied. This model is obtained by applying a vertical magnetic field
perpendicular to the plane of one dimensional ring of particles. The
trigonometric form of the Hamiltonian is recast by using a suitable similarity
transformation. The transformed Hamiltonian is shown to be integrable by
constructing a set of momentum operators which commutes with the Hamiltonian
and amongst themselves. The function space of monomials of several variables
remains invariant under the action of these operators. The above properties
imply the quasi-solvability of the Hamiltonian under consideration.
