### Introduction

• Structured representations for weight help achieve good compression and acceleration while maintaining generalization properties.

• Some popular examples of low-rank matrices are: Block circulant, Toeplitz matrices, etc

• These constructions tend to underperform on general fully-connected layers

• The proposed approach leverages a LDR (low displacement rank) framework, which encodes structure through two sparse displacement operators and a low-rank residual term.

• Displacement operators are jointly learned with the low rank residual.

• Achieves higher accuracy than the general unstructured layers while using an order of magnitude fewer parameters.

• Provides guaranteed compression, matrix-vector multiplication algorithms that are quasi-linear in number of parameters.

### Displacement Operators and LDR Framework

• Represent a matrix $\mathbf{M} \in \mathbb{R}^{m \times n}$ through displacement operators $\left(\mathbf{A} \in \mathbb{R}^{m \times m}, \mathbf{B} \in \mathbb{R}^{n \times n}\right)$

• $\nabla_{\mathbf{A}, \mathbf{B}} : \mathbf{M} \mapsto \mathbf{A M}-\mathbf{M B}$ with a residual $\mathbf{R}$ so that $\mathbf{AM - MB = R}$

• $M$ can be manipulated through compressed representation $\mathbf{(A, B, R)}$

• Assuming $A$ and $B$ have disjoint eigenvalues, $\mathbf{M}$ can be recovered from $\mathbf{(A, B, R)}$

• Rank $\mathbf{R}$ is called the displacement rank of $\mathbf{M}$ w.r.t $\mathbf{(A, B)}$

• Toeplitz-like matrices have LDR w.r.t shift or cycle operators.

• Standard formulation for $\mathbf{A}$ and $\mathbf{B}$

$\mathbf{A}= \mathbf{Z}_{1}$, $\mathbf{B}= \mathbf{Z}_{-1}$ where $\mathbf{Z}_{f}= \begin{bmatrix} \hspace{-0.3cm} 0_{1 \hspace{0.05cm} \times \hspace{0.05cm} (n-1)} \hspace{0.6cm} f \\ \hspace{0.1cm} I_{n-1} \hspace{0.6cm} 0_{(n-1) \hspace{0.05cm} \times \hspace{0.05cm} 1} \end{bmatrix}$

• Displacement equation for Toeplitz like matrix.

• Shift invariant structure leads to a residual rank of atmost 2.

• Some examples of matrices that satisfy displacement property:Hankel, Toeplitz, Cauchy, etc.

• Certain combinations of LDR matrices preserve low displacement rank

### Learning Displacement Operators

• Two classes of new displacement operators: Sub-diagonal and Tri-Diagonal

• These matrices can model rich structure and subsume many types of linear transformations used in machine learning

• Given $\mathbf{A, B, R}$, we need to do a matrix-vector multiplication.

• Several schemes for explicitly reconstructing $\mathbf{M}$ from its displacement parameters are known for specific cases but do not always apply to general operators.

• $\mathbf{(A, B, R)}$ are used to implicitly construct a slightly different matrix with at most double the displacement rank, which is simpler to work with.

• $\mathbf{(A, B, G, H)}$ are stored, $\mathbf{(A, B)}$ are either sub-diagonal or tri-diagonal operators and $\mathbf{(G, H) \in \mathbb{R}^{n \times r}}$

• Near linear time algorithms for LDR-TD and LDR-SD are recently shown to exist.

• Complete algorithms were not provided, as they relied on theoretical results such as the transposition principle.

• Paper provides a near-linear time algorithm for LDR-SD matrices.

• For LDR-TD, $\mathcal{K}\left(\mathbf{A}, \mathbf{g}_{i}\right) \text { and } \mathcal{K}\left(\mathbf{B}^{T}, \mathbf{h}_{i}\right)$ are explicitly constructed and then standard matrix vector multiplication is used.

### Theoretical Properties of Structured Matrices

• The matrices used are unusual in that the parameters interact multiplicatively (namely in $\mathbf{A_i}$, $\mathbf{B_i}$) to implicitly define the actual layer.

• It can be proved that this does not change the learnability of classes.

• Displacement rank and equivariance: Related to building equivariant feature representations that transform in predictable ways when the input is transformed. An equivariant feature map $\Phi$ satisfies:

$\Phi(B(x))=A(\Phi(x))$

• Perturbing the input by a transformation B before passing through the map $\Phi$ is equivalent to first finding the features $\Phi$, then transforming by $A$.

• LDR matrices are a suitable choice for modeling approximately equivariant linear maps.

### Results

• The proposed method is tested on a single hidden layer neural network and the fully-connected layer of a CNN.

• Datasets used: Two variants of MNIST (bg-rot, noise), CIFAR-10 (grayscale), and NORB.

• The table shown below compares the proposed approach to several baselines using dense, structured matrices to compress the hidden layer of a single hidden layer neural network.

• Paper also shows results for language modeling task by replacing the weights in a single layer LSTM

• Results on replacing a five-layer CNN consisting of convolutional channels, max pooling, FC layers with two generic LDR matrices.

### References:

1. Thomas, A., Gu, A., Dao, T., Rudra, A., and Ré, C. Learning compressed transforms with low displacement rank. In Bengio, S., Wallach, H., Larochelle, H., Grauman, K., Cesa-Bianchi, N., and Garnett, R. (eds.), Advances in Neural Information Processing Systems 31, pp. 9066–9078. Curran Associates, Inc., 2018