In the case of the crew pairing problem, negative
reduced cost columns are identified without explicitly considering each pairing. Instead, all columns
are implicitly considered by solving a pricing problem, often formulated as a multilabel, shortest path
problem in which some paths correspond to pairings.
By exploiting dominance, shortest paths—and hence
minimum reduced cost pairings—are identified without examining all paths, or pairings. For a detailed
exposition of multilabel, shortest path problems, see
Desrochers and Soumis (1988).