The following is a list of texts, which I am looking for. If you have a copy. I would be grateful, if you could send me one.

• G.L.Nemhauser, L.A.Wolsey, M.L.Fisher, An Analysis of the Approximations for Maximizing Submodular Set Functions. Mathematical Programming, 14:265-294, 1978.
• Adamek, Trnkova, Automata and Algebras in Categories, Math. and its Appl., Kluwer, 1989
• Diers, Categories multialgebraiques, Arch. Math. (Basel) (1980) 193-209