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Title: Gradient Descent Maximizes the Margin of Homogeneous Neural Networks

Paper content:
In this paper, we study the implicit regularization of the gradient descent algorithm in homogeneous neural networks, including fully-connected and convolutional neural networks with ReLU or LeakyReLU activations.
In particular, we study the gradient descent or gradient flow (i.e., gradient descent with infinitesimal step size) optimizing the logistic loss or cross-entropy loss of any homogeneous model (possibly non-smooth), and show that if the training loss decreases below a certain threshold, then we can define a smoothed version of the normalized margin which increases over time.
We also formulate a natural constrained optimization problem related to margin maximization, and prove that both the normalized margin and its smoothed version converge to the objective value at a KKT point of the optimization problem.
Our results generalize the previous results for logistic regression with one-layer or multi-layer linear networks, and provide more quantitative convergence results with weaker assumptions than previous results for homogeneous smooth neural networks.
We conduct several experiments to justify our theoretical finding on MNIST and CIFAR-10 datasets.
Finally, as margin is closely related to robustness, we discuss potential benefits of training longer for improving the robustness of the model.
A major open question in deep learning is why gradient descent or its variants, are biased towards solutions with good generalization performance on the test set.
To achieve a better understanding, previous works have studied the implicit bias of gradient descent in different settings.
One simple but insightful setting is linear logistic regression on linearly separable data.
In this setting, the model is parameterized by a weight vector w, and the class prediction for any data point x is determined by the sign of w x.
Therefore, only the direction w/ w 2 is important for making prediction.
Soudry et al. (2018a; b) ; Ji and Telgarsky (2018) ; Nacson et al. (2018) investigated this problem and proved that the direction of w converges to the direction that maximizes the L 2 -margin while the norm of w diverges to +∞, if we train w with (stochastic) gradient descent on logistic loss.
Interestingly, this convergent direction is the same as that of any regularization path: any sequence of weight vectors {w t } such that every w t is a global minimum of the L 2 -regularized loss L(w) + with λ t → 0 (Rosset et al., 2004) .
Indeed, the trajectory of gradient descent is also pointwise close to a regularization path (Suggala et al., 2018) .
The aforementioned linear logistic regression can be viewed as a single-layer neural network.
A natural and important question is to what extent gradient descent has similiar implicit bias for modern deep neural networks.
For theoretical analysis, a natural candidate is to consider homogeneous neural networks.
Here a neural network Φ is said to be (positively) homogeneous if there is a number L > 0 (called the order) such that the network output Φ(θ; x), where θ stands for the parameter and x stands for the input, satisfies the following:
∀c > 0 : Φ(cθ; x) = c L Φ(θ; x) for all θ and x.
(1) It is important to note that many neural networks are homogeneous (Neyshabur et al., 2015; Du et al., 2018) .
For example, deep fully-connected neural networks or deep CNNs with ReLU or LeakyReLU activations can be made homogeneous if we remove all the bias terms, and the order L is exactly equal to the number of layers.
In (Wei et al., 2018) , it is shown that the regularization path does converge to the max-margin direction for homogeneous neural networks with cross-entropy or logistic loss.
This result suggests that gradient descent or gradient flow may also converges to the max-margin direction by assuming homogeneity, and this is indeed true for some sub-classes of homogeneous neural networks.
For gradient flow, this convergent direction is proven for linear fully-connected networks (Ji and Telgarsky, 2019a) .
For gradient descent on linear fully-connected and convolutional networks, (Gunasekar et al., 2018b ) formulate a constrained optimization problem related to margin maximization and prove that gradient descent converges to the direction of a KKT point or even the max-margin direction, under various assumptions including the convergence of loss and gradient directions.
In an independent work, (Nacson et al., 2019a) generalize the result in (Gunasekar et al., 2018b) to smooth homogeneous models (we will discuss this work in more details in Section 2).
In this paper, we analyze the dynamics of gradient flow/descent of homogeneous neural networks under a minimal set of assumptions.
The main technical contribution of our work is to prove rigorously that for gradient flow/descent, the normalized margin is increasing and converges to a KKT point of a natural max-margin problem.
Our results leads to some natural further questions:
• Can we generalize our results for gradient descent on smooth neural networks to nonsmooth ones?
In the smooth case, we can lower bound the decrement of training loss by the gradient norm squared, multiplied by a factor related to learning rate.
However, in the non-smooth case, no such inequality is known in the optimization literature, and it is unclear what kind of natural assumption can make it holds.
• Can we make more structural assumptions on the neural network to prove stronger results?
In this work, we use a minimal set of assumptions to show that the convergent direction of parameters is a KKT point.
A potential research direction is to identify more key properties of modern neural networks and show that the normalized margin at convergence is locally or globally optimal (in terms of optimizing (P)).
• Can we extend our results to neural networks with bias terms?
In our experiments, the normalized margin of the CNN with bias also increases during training despite that its output is non-homogeneous.
It is very interesting (and technically challenging) to provide a rigorous proof for this fact.

assistant: text: We study the implicit bias of gradient descent and prove under a minimal set of assumptions that the parameter direction of homogeneous models converges to KKT points of a natural margin maximization problem.