  A method is presented to obtain the canonical-form solutions of the HFB
equation for atomic nuclei with zero-range interactions like the Skyrme force.
It is appropriate to describe pairing correlations in the continuum in
coordinate-space representations. An improved gradient method is used for
faster convergences under constraint of orthogonality between orbitals. To
prevent high-lying orbitals to shrink into a spatial point, a repulsive
momentum dependent force is introduced, which turns out to unveil the nature of
high-lying canonical-basis orbitals. The asymptotic properties at large radius
and the relation with quasiparticle states are discussed for the obtained
canonical basis.
