ReLU, or the rectified linear unit, maps an input x to max(0, x). It outputs zero for negative inputs and preserves positive inputs, adding nonlinearity after convolutional layers. In VGG, repeated ReLU activations make stacked 3 × 3 convolutions more expressive. section 11.2.1
Its practical behavior in convolutional networks includes:
- Negative activations become zero, while positive activations pass through unchanged. section 11.2.1
- VGG places a ReLU after each convolution, increasing nonlinearity without changing the convolution's spatial resolution. section 11.2.1
- ReLU is unbounded; unlike local response normalization, it does not bound large outputs. section 11.2.1
PyTorch inside a convolutional block
A VGG-style block pairs each 3 × 3 convolution with nn.ReLU(inplace=True), then applies max pooling after the block. The same activation also appears in the fully connected classifier. section 11.2.1
Equation 11.1
| ReLU(x) = max(0, x) |
Use the equation to verify the zero-for-negative and identity-for-positive behavior. section 11.2.1











