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Nowadays, networks (systems) appear in many areas of science and technology. One of the most important strategies to reduce the likelihood of the failure of an operating network is preventive maintenance (PM). In this article, we propose some optimal PM models for a network consisting of $n$, $n \geq 1$, links (components). The criteria of interest are the `cost function' of renewing the network and `stationary availability' of the network during its mission. In the first part of the paper, we consider the case that the network is formed of identical components while in the second part, we deal with the case that the network is built up from several non-identical groups of components. In both parts, we utilize the PM under some partial information about the status of the network at a time $t$. The results of the paper are developed using the notions of signature and survival signature. To interpret the proposed models, the results are illustrated numerically and graphically for two networks.
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