When n storage types are heterogeneous and upscaling oper-
ations are allowed, Kolen, Lenstra, and Papadimitriou (1987)
show that the problem is strongly NP-hard if there are three
or more storage types. They employ a reduction from “nu-
merical 3-dimensional matching.” Kolen and Kroon (1991)
further show that the problem still is strongly NP-hard if
there are only two storage types. The reduction is also
from “numerical 3-dimensional matching.” Carter and Tovey
(1992) discuss problem variants in classroom assignments.