  A systematic and easy-to-use method is developed to calculate directly the
doubly heavy hadron spectral density in the coordinate space. The correlation
function is expressed in terms of hypergeometric functions, and the spectral
density is obtained through two independent approaches: the simple integral
representation method and the epsilon-expansion method, respectively. It is
found that the spectral density of doubly heavy hadrons can be analytically
expressed through commonly known simple functions. This method can drastically
simplify and improve the QCD spectral sum rule calculation of the doubly heavy
hadrons. An instructive numerical method is also presented for fast evaluation
of the spectral density.
