Duplication is a technique frequently employed to achieve high reliability for a repairable system. Although the philosophy of duplication is that it takes two faults to place a system out of service, there are generally some critical single faults that cause a system failure. This paper considers the effect of such a set of faults on a repairable system's reliability. It is shown that even a small number of such faults may severely degrade the mean time to system failure and the expected downtime for an otherwise highly reliable system.

Universal modules are presented that can be used to realize any single input-single output synchronous sequential machine in the form of a circuit in which the input signal enters only one module. Additionally, no signal in the circuit fans out to more than some bounded number of terminals where the bound depends only on the module being used. Two different realization schemes are presented. The first scheme is based on the single feedback loop circuit model. Modules are connected in a highly regular array. Inputs to the last row of the array are constant logic signals. In the second scheme, a tree of modules is connected at the back to one or more one-hot reaiizations of a machine closely related to the machine being realized. The first scheme has the advantage of yielding a circuit with a highly regular interconnections pattern. The second scheme generally requires fewer modules. The theory is easily edxtended to machines with more than one input and output.

This paper is concerned with the recognition of words which are contained in languages defined by pushdown store machines. Such languages exhibit many of the syntactic properties of algorithmic programming languages. Thus the recognition problem for languages generated by pushdown store machines is related to compilation of programming languages.

Synthesis techniques are presented for realizing an arbitrary synchronous flow table in the form of an array of identical modules interconnected in a regular pattern. Several types of structures and their corresponding modules are considered, and a relationship between these structures and earlier work on combinational circuits is shown.