A ReLU, or rectified linear unit, is an activation function: it replaces every negative input with 0 and leaves positive values unchanged. This simple rule adds nonlinearity to a neural network. section 11.25
ReLU is useful because some neurons can remain inactive instead of all activating at once. That can reduce computation and may help the network find solutions faster, unlike activation functions whose extreme values can contribute to vanishing gradients. section 9.10
Applying ReLU step by step
Given the vector [1.5921, 0.3436, -0.5260, 1.7674, -0.4769], ReLU scans each value independently. The positive values stay the same, while the negative values become zero, producing [1.5921, 0.3436, 0, 1.7674, 0]. In a projection layer, this happens after multiplying by weights and adding a bias. section 11.25











