The Multi-Vehicle Traveling Purchaser Problem with Pairwise Incompatibility Constraints and Unitary Demands: A Branch-and-Price Approach

In this work we study a suppliers selection and routing problem where a fleet of
homogeneous vehicles with a predefined capacity is available for procuring different products from
different suppliers with the aim to minimize both the traveling and the purchasing costs. Decisions are
further complicated by the presence of pairwise incompatibility constraints among products, implying
the impossibility of loading two incompatible products on the same vehicle. The problemis known as
the Multi-Vehicle Traveling Purchaser Problem with Pairwise Incompatibility Constraints.We study a
variant in which products demand is unitary and propose a column generation approach based on a
Dantzig-Wolfe reformulation of the problem, where each column represents a feasible vehicle route
associated with a compatible purchasing plan.Two different procedures are introduced to solve the
pricing problem, namely a labeling algorithm solving a Resource-Constrained Elementary Shortest
Path Problem on an expanded graph, and a tailored branch-and-cut algorithm. Due to the integrality
request on variables, we embed the column generation in a branch-and-bound framework and propose
different branching rules, thus obtaining a branch-and-price procedure. Extensive tests, carried out on
a large set of instances, show that our branch-and-price method performs well, improving on average,
both in quality and in computational time, solutions obtained by a branch-and-cut approach existing in
the literature that relies on a three-index connectivity constraints based formulation.