### Description

*n*-component system contains

*n*components, where each component may be either failing or working. Each component

*i*is failing with probability

*Pi*independently of the other components in the system. A system state is a specification of the states of the

*n*components. Let

*F*, the set of failure states, be a specified subset of all system states. In this paper we develop Monte Carlo algorithms to estimate

*Pr*[

*F*], the probability that the system is in a failure state.

We now describe the two different formats for the representation of *F* considered in this paper. In the first format *F* is represented by *m* failure sets *F1*, *F2*, . . . , *Fm*. Each failure set is specified by an *n*-tuple in the input. The set of failure states is then *F*=m,t=1*Fi*. We develop a formula for the probability of a union of events. A Monte Carlo algorithm which estimates the failure probability of an *n*-component system is developed based on this formula. One trial of the algorithm outputs an unbiased estimator of *Pr*[*F*]. Let *Y* be the average of the estimators produced by many trials of the algorithm. We show that, when the algorithm is run for an amount of time proportional to *nm*, *Y* is provably close to *Pr*[*F*] with high probability.

The second format for the representation of *F* can be described as follows. A network is an undirected graph *G*, where the edges in the graph correspond to the components in the system. Let *G* be a planar network and let *x1*, . . . , *xK* be *K* specified nodes in *G*. For the planar *K*-terminal problem, the network is in a failing state if there is no path of working edges between some pair of specified nodes. We develop a Monte Carlo algorithm to estimate *Pr*[*F*] for the planar *K*-terminal problem. The algorithm works especially well when the edge failure probabilities are small. In this case the algorithm produces an estimator *Y* which is provably close to *Pr*[*F*] with high probability in time polynomial in the size of the graph. This compares very favorably with the execution times of other methods used for solving this problem.