Premise: The incidence matrix of the PRDG is TUM.
To prove: either there exists a null-weight cycle or there exists a multi-dimensional time schedule
let the weight of an edge e_i be given by w(e_i) \in \mathcal(Z)^d
(we will talk about the unitarization case later)
notes from schrijver-notes
Interval graphs are TUM
Interval graphs are one more special cases of network matrices (directed graphs are the other special case of network matrices)