  Two-point tree-level amplitudes in bosonic closed string theory are described
by a correlation function within the BRST formalism, which respects manifest
Lorentz and conformal invariance. In the derivation of the two-point
amplitudes, we use the mostly BRST exact operator, which has been introduced
for two-point open string amplitudes, and a closed string vertex with ghost
number three, which has been explored in our recent work, in addition to the
conventional one.
