Semiretracts : a counterexample and some results

AbstractIn the paper (Theoret. Comput. Sci. 237 (2000)) Anderson present a theorem which characterizes any semiretract S by means of two retracts Rα and Rω. The first part of the paper contains a counterexample for this characterization. Then some results are presented which finally lead to the theorem which determines for a given semiretract S the minimal number of retracts R1,…,Rm such that the equality S=⋂i=1mRi holds