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This paper deals with the calculations in the reasoning process of rulebased expert systems, where inference chains are applied. It presents a logic model for representing the rules and the rule base of a given system. Also, the fact base of the same expert system is involved in the logic model. The proposed equivalent representation manifests itself in a logic network. After that, a four-valued logic algebra is introduced. This algebra is used for the calculatíons where forward chaining is carried out. Next, the notion oflinevalue justification is described. This operation is applied in the backward chailling process, also on the base of the previously introduccd four-valued logic. The paper describes two exact algorithrns which serve for the forward and backward chaining processes. These algorithms can be implemented by a computer program, resulting in an efficient inference engine of an expert system. The achieved result enhances the reliability and usabili ty of the intelligent software systems which is extremely important in embedded environments.
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