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The problem of subcarrier, bit and power allocation for an OFDM based cognitive radio system in which one or more spectrum holes exist between multiple primary user (PU) frequency bands is studied. The cognitive radio user is able to use any portion of the frequency band as long as it does not interfere unduly with the PUs' transmissions. We formulate the resource allocation as a multidimensional knapsack problem and propose a low-complexity, greedy max-min algorithm to solve it. The proposed algorithm is simple to implement and simulation results show that its performance is very close to (within 0.3% of) the optimal solution.