  For the one-dimensional XXX model under the periodic boundary conditions, we
discuss two types of eigenvectors, regular eigenvectors which have
finite-valued rapidities satisfying the Bethe ansatz equations, and non-regular
eigenvectors which are descendants of some regular eigenvectors under the
action of the SU(2) spin-lowering operator. It was pointed out by many authors
that the non-regular eigenvectors should correspond to the Bethe ansatz
wavefunctions which have multiple infinite rapidities. However, it has not been
explicitly shown whether such a delicate limiting procedure should be possible.
In this paper, we discuss it explicitly in the level of wavefunctions: we prove
that any non-regular eigenvector of the XXX model is derived from the Bethe
ansatz wavefunctions through some limit of infinite rapidities. We formulate
the regularization also in terms of the algebraic Bethe ansatz method. As an
application of infinite rapidity, we discuss the period of the spectral flow
under the twisted periodic boundary conditions.
