Szukana fraza: [Abstract = "A high number of Internet shops makes it difficult for a customer to review manually all the available offers and select optimal outlets for shopping. A partial solution to the problem is brought by price comparators which produce price rankings from collected offers. However, their possibilities are limited to a comparison of offers for a single product requested by the customer. The issue we investigate in this paper is a multiple\-item multiple\-shop optimization problem, in which total expenses of a customer to buy a given set of items should be minimized over all available offers. In this paper, the Internet Shopping Optimization Problem \(ISOP\) is defined in a formal way and a proof of its strong NP\-hardness is provided. We also describe polynomial time algorithms for special cases of the problem."]