High order perturbation theory for spectral densities of multi-particle excitations: S = 1/2 two-leg Heisenberg ladderC. Knetter, K. P. Schmidt and G. S. UhrigEur. Phys. J. B, 36 4 (2003) 525-544DOI: https://doi.org/10.1140/epjb/e2004-00008-2