So I was reading this paper: https://arxiv.org/pdf/1409.1556.pdf
VERY DEEP CONVOLUTIONAL NETWORKS FOR LARGE-SCALE IMAGE RECOGNITION Karen Simonyan∗ & Andrew Zisserman+ Visual Geometry Group, Department of Engineering Science, University of Oxford
and at a point it mentions: "It is easy to see that a stack of two 3 × 3 conv. layers (without spatial pooling in between) has an effective receptive field of 5 × 5; three such layers have a 7 × 7 effective receptive field."
I don't understand how these effective receptive fields are calculated in relation to the convolutions / convolution units.