  Modifying the conventional antiferromagnetic spin-wave theory which is
plagued by the difficulty of the zero-field sublattice magnetizations diverging
in one dimension, we describe magnetic properties of Haldane-gap
antiferromagnets. The modified spin waves, constituting a grand canonical
bosonic ensemble so as to recover the sublattice symmetry, not only depict well
the ground-state correlations but also give useful information on the
finite-temperature properties.
