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In this work we consider the relay node placement problem in wireless sensor networks: where to position a limited number of available nodes that can act as additional relays to foward sensor data toward one or more base stations. The objective is to define relay node locations in order to improve network performance in terms of delivery ratio and end-to-end delay, and/or to provide connectivity in partially disconnected areas. Typical application scenarios include the repair of the network in face of failures, or the case of networks used in dynamic environments, such as network characteristics need to be dynamically adapted to the changing conditions. We formalize the problem by defining a linear, mixed integer mathematical programming model. We include a number of constraints and penalty components, aimed at closely modeling the specific characteristics of the wireless environment. Model solutions specify both where to place the relays and the optimal data paths to route the data. Through a comprehensive evaluation in simulation we show that our approach is effective in accomplishing the desired objectives. We compare the solutions provided by our scheme against a state-of-the-art dynamic routing protocol, to assess the quality of the routes, and against a relay node placement heuristic, to evaluate the positioning of the relay nodes.