  A Fourier-Matsubara series expansion is derived for imaginary-time
correlation functions that constitutes the imaginary-time generalization of the
infinite Matsubara series for equal-time correlation functions. The expansion
is consistent with all known exact properties of imaginary-time correlation
functions and opens up new avenues for the utilization of quantum Monte Carlo
simulation data. Moreover, the expansion drastically simplifies the computation
of imaginary-time density-density correlation functions with the finite
temperature version of the self-consistent dielectric formalism. Its existence
underscores the utility of imaginary-time as a complementary domain for
many-body physics.
