  We numerically study the magnetization and the dispersion relation of a
frustrated quantum spin system. Our method, which is named the stochastic state
selection method, is a kind of Monte Carlo method to give eigenstates of the
system through statistical averaging processes.
  Using the stochastic state selection method with some constraints, we make a
successful study of the spin-1/2 Heisenberg antiferromagnet on a 48-site
triangular lattice. We calculate the sublattice magnetization and the static
structure function in the ground state. Our result on the sublattice
magnetization is consistent with the value given by the linear spin wave
theory. This adds an evidence for the analysis based on the spontaneous
symmetry breaking of the semi-classical Neel order in the ground state.
  We also evaluate the low-lying one magnon spectra of the model with all wave
vectors available on a 48-site triangular lattice. We find that at the ordering
wave vector there is a Goldstone mode, which is in good agreement with the
result from the spin wave analysis. The magnon spectra with other wave vectors,
however, are quite different from results obtained by the linear spin wave
theory. We observe a flat dispersion relation with a strong downward
renormalization. Our results are compatible with those recently reported in the
series expansion study and in the order 1/S calculation of the spin wave
analysis.
