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We consider the problem of resource allocation and relay selection in an Orthogonal Frequency Division Multiple Access (OFDMA) based downlink network. We assume the availability of multiple cooperative relay stations but not the perfect channel state information (CSI). Instead, the base station knows only the estimated channel gain and the associated error distribution. Using a tight capacity lower bound (CLB) as the performance metric, we derive the optimal source and relay power allocation ratio that maximizes the CLB of a cascaded source-relay-destination link. Based on this power ratio, we propose a simple suboptimal algorithm that assigns power, subcarriers and cooperative relays to each mobile station. Simulation results show that our algorithm not only meets the users' rate constraints with very high probabilities but yields an excellent sum rate (CLB) performance.