The Internet Shopping Optimization Problem with Multiple Item Units (ISHOP-U): Formulation, Instances, NP-Completeness, and Evolutionary Optimization oleh Fernando Ornelas, Alejandro Santiago, Salvador Ibarra Martínez, Mirna Patricia Ponce-Flores, Jesús David Terán-Villanueva, Fausto Balderas, José Antonio Castán Rocha, Alejandro H. García, Julio Laria-Menchaca, Mayra Guadalupe Treviño-Berrones