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代码说明：

说明：  BP算法是为了解决多层前向神经网络的权系数优化而提出来的；所以，BP算法也通常暗示着神经网络的拓扑结构是一种无反馈的多层前向网络(BP algorithm is to solve the multi-layer feedforward neural network weights optimization and put forward Therefore, BP algorithm usually implies that the topology of neural network is a multi-layer feedback-free network)