  The spin Green's function of the antiferromagnetic Heisenberg model on a
triangular lattice is calculated using Mori's projection operator technique. At
T=0 the spin excitation spectrum is shown to have gaps at the wave vectors of
the classical Neel ordering. This points to the absence of the
antiferromagnetic long-range order in the ground state. The calculated spin
correlation on the neighboring sites of the same sublattice is in good
agreement with the value derived from exact diagonalization. The temperature
dependencies of the spin correlations and the gaps are calculated.
