system reliability; unit reliability; unit importance; essential unit
The availability of a system with dependent units is obtained in the case where the system fails when one of the essential units fails. Markov model is assumed. The system considered consists of $n$ dependent units of which $r\leq n$ units are essential units. A unit is said to be essential if its failure causes the system to fail. The mean and variance of time to system failure are given. Unit reliability is also discussed.
 John G. Kemeny J. Laurie Snell: Finite Markov Chain
. D. Van Nostrand Company Inc., New York (1960). MR 0115196