Traveling purchaser problem
The traveling purchaser problem (TPP) is an NP-hard problem studied in theoretical computer science. Given a list of marketplaces, the cost of travelling between different marketplaces, and a list of available goods together with the price of each such good at each marketplace, the task is to find for a given list of articles the route with the minimum combined cost of purchases and traveling. The traveling salesman problem (TSP) is a special case of this problem.
Relation to TSP
The problem can be seen as a generalization of the traveling salesman problem, ie. each article is available at one market only and each market sells only one item. Since TSP is NP-hard, TPP is NP-hard.