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I have been reading through Chapter 9 of www.deeplearningbbook.org, where convolutional networks are being described.

The following image represents the output of a 2D convolution, without kernel flipping.

enter image description here

The book goes on to describe this matrix as a Toeplitz matrix where,

for univariate discrete convolution, each row of the matrix is constrained to be equal to the row above shifted by one element.

I fully understand this statement since w, x, y and z are constants in their respective columns with shifting elements.

However, there is no mention of diagonal-constants which are a key feature of such matrices. As per Wikipedia (link above) and several other sources:

a Toeplitz matrix or diagonal-constant matrix, named after Otto Toeplitz, is a matrix in which each descending diagonal from left to right is constant.

Going back to the image above, does this mean that, for instance, aw == cx == gy? How can this be ensured when all elements are different?

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The matrices in that picture are not toeplitz matrices. Rather, convolution can be treated as matrix multiplication by a toeplitz matrix.

For example, to convolve $x = [1, 3, 1, 0]^T$ by the filter [5, 1, 1], you first construct the toeplitz matrix:

$$M = \begin{bmatrix} 5 & 1 & 1 & 0 & 0 & 0\\ 0 & 5 & 1 & 1 & 0 & 0\\ 0 & 0 & 5 & 1 & 1 & 0\\ 0 & 0 & 0 & 5 & 1 & 1\\ \end{bmatrix}$$

Then $Mx$ computes the convolution (technically correlation!) of $x$ by the filter. Clearly $M$ is toeplitz.

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