  We prove an abstract interpolation theorem which interpolates the
(r,2)-summing and (s,2)-mixing norm of a fixed operator in the image and the
range space. Combined with interpolation formulas for spaces of operators we
obtain as an application the original Bennett-Carl inequalities for identities
acting between Minkowski spaces l_u as well as their analogues for Schatten
classes S_u. Furthermore, our techniques motivate a study of Bennett-Carl
inequalities within a more general setting of symmetric Banach sequence spaces
and unitary ideals.
