Arvind Gupta Pavol Hell Mehdi Karimi Arash Rafiey Minimum Cost Homomorphisms to Reflexive Digraphs. 182-193 2008 LATIN http://dx.doi.org/10.1007/978-3-540-78773-0_16 conf/latin/2008 db/conf/latin/latin2008.html#GuptaHKR08