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Cooperative diversity is a promising technology for future wireless networks. In this paper, we derive the average bit error rate (BER) and outage probability (Pout) for differential equal gain combining (EGC) in cooperative diversity networks. The considered network uses adaptive decode-and-forward (DF) relaying over independent non-identical Nakagami-m fading channels. In adaptive DF relaying among M relays, that can participate, only C relays (C les M), with good channels to the source, decode and then forward (retransmit) the source information to the destination. Then, the destination combines the direct and the indirect signals using differential EGC. We first derive a simple exact expression for the equivalent SNR at the destination. Second, we derive the expressions of the PDF and the MGF of this equivalent total SNR at the destination. Then the MGF is used to determine the error and outage probabilities of adaptive DF with an arbitrary number of relays. Furthermore, we found (in terms of MGF) the SNR moments, the average signal-to-noise ratio (SNR) and the amount of fading. Computer simulations are used to validate our analytical results. Results show the significant performance improvement due to the use of the adaptive DF cooperative diversity. Also, results show that the performance of the adaptive DF differential EGC is comparable to the adaptive DF maximum ratio combining (MRC) performance.