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Lee et al. propose a regularizer to increase the size of linear regions of rectified deep networks around training and test points. Specifically, they assume piecewise linear networks, in its most simplistic form consisting of linear layers (fully connected layers, convolutional layers) and ReLU activation functions. In these networks, linear regions are determined by activation patterns, i.e., a pattern indicating which neurons have value greater than zero. Then, the goal is to compute, and later to increase, the size $\epsilon$ such that the $L_p$ball of radius $\epsilon$ around a sample $x$, denoted $B_{\epsilon,p}(x)$ is contained within one linear region (corresponding to one activation pattern). Formally, letting $S(x)$ denote the set of feasible inputs $x$ for a given activation pattern, the task is to determine $\hat{\epsilon}_{x,p} = \max_{\epsilon \geq 0, B_{\epsilon,p}(x) \subset S(x)} \epsilon$. For $p = 1, 2, \infty$, the authors show how $\hat{\epsilon}_{x,p}$ can be computed efficiently. For $p = 2$, for example, it results in $\hat{\epsilon}_{x,p} = \min_{(i,j) \in I} \frac{z_j^i}{\\nabla_x z_j^i\_2}$. Here, $z_j^i$ corresponds to the $j$th neuron in the $i$th layer of a multilayer perceptron with ReLU activations; and $I$ contains all the indices of hidden neurons. This analytical form can then used to add a regularizer to encourage the network to learn larger linear regions: $\min_\theta \sum_{(x,y) \in D} \left[\mathcal{L}(f_\theta(x), y)  \lambda \min_{(i,j) \in I} \frac{z_j^i}{\\nabla_x z_j^i\_2}\right]$ where $f_\theta$ is the neural network with paramters $\theta$. In the remainder of the paper, the authors propose a relaxed version of this training procedure that resembles a maxmargin formulation and discuss efficient computation of the involved derivatives $\nabla_x z_j^i$ without too many additional forward/backward passes. https://i.imgur.com/jSc9zbw.jpg Figure 1: Visualization of locally linear regions for three different models on toy 2D data. On toy data and datasets such as MNIST and CalTech256, it is shown that the training procedure is effective in the sense that larger linear regions around training and test points are learned. For example, on a 2D toy dataset, Figure 1 visualizes the linear regions for the optimal regularizer as well as the proposed relaxed version. Also find this summary at [davidstutz.de](https://davidstutz.de/category/reading/).
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