Haggkvist, RolandHell, PavolMiller, Donald J.Neumann Lara, Victor2009-08-252009-08-2519852009-08-25http://hdl.handle.net/1828/1589We characterize those directed and undirected paths P and cycles C which have the property that the class of graphs not admitting a homomorphism into P, respectively C, is closed under taking the product (conjunction). Whether all undirected complete graphs have the property is a longstanding open question due to S. Hedetniemi.enTechnical Report