  In most non-perturbative methods in hadron physics the calculations are
started with a correlation function in terms of some interpolating and
transition currents in $ x $-space. For simplicity, the calculations are then
transformed to the momentum space by a Fourier transformation. To suppress the
contributions of the higher states and continuum; and enhance the ground state
contribution, Borel transformation as well as continuum subtraction are applied
by the help of quark-hadron duality assumption. In the present study we work
out the mathematics required for these processes in the case of light and
multi-heavy hadrons. We address a well-known problem in subtraction of the
effects of the higher states and continuum and discuss how we find finite
results without any divergence by using an appropriate representation of the
modified Bessel functions, appearing in the heavy quark propagator, and
successive applications of the Borel transformations, which lead to more
suppression of the higher states and continuum contributions. The results
obtained can be used in determination of the spectroscopic and decay properties
of the multi-heavy standard and non-conventional (exotic) systems in many
non-perturbative methods, specially the QCD sum rules.
