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Fundamental to the usefulness of any device, instrument or system are the initial verification of its correct functioning and the maintenance of its performance during ing operation. The following discussion is concerned with such problems of initial verification and maintenance for digital systems. Systems can fail to function properly either because of incorrect specification, design, or implementation; or else because of physical deterioration or damage. Problems caused by incorrect specification are extremely difficult to deal with formally. Many cases of faulty design or implementation can be detected either by formal analysis techniques (e.g., logic circuit analysis) or by simulation methods. Of course, actual physical implentations such as integrated circuits must be subjected to measurement before final acceptance. This process of measuring the output response with specified signals applied to the inputs is called testing and is useful both for initial acceptance and also for verification of continued validity during use.