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Title: A Functional Characterization of Randomly Initialized Gradient Descent in Deep ReLU Networks

Paper content:
Despite their popularity and successes, deep neural networks are poorly understood theoretically and treated as 'black box' systems.
Using a functional view of these networks gives us a useful new lens with which to understand them.
This allows us us to theoretically or experimentally probe properties of these networks, including the effect of standard initializations, the value of depth, the underlying loss surface, and the origins of generalization.
One key result is that generalization results from smoothness of the functional approximation, combined with a flat initial approximation.
This smoothness increases with number of units, explaining why massively overparamaterized networks continue to generalize well.
Deep neural networks, trained via gradient descent, have revolutionized the field of machine learning.
Despite their widespread adoption, theoretical understanding of fundamental properties of deep learning -the true value of depth, the root cause of implicit regularization, and the seemingly 'unreasonable' generalization achieved by overparameterized networks -remains mysterious.
Empirically, it is known that depth is critical to the success of deep learning.
Theoretically, it has been proven that maximum expressivity grows exponentially with depth, with a smaller number of trainable parameters (Raghu et al., 2017; Poole et al., 2016) .
This theoretical capacity may not be used, as recently shown explicitly by (Hanin & Rolnick, 2019) .
Instead, the number of regions within a trained network is proportional to the total number of hidden units, regardless of depth.
Clearly deep networks perform better, but what is the value of depth if not in increasing expressivity?
Another major factor leading to the success and widespread adoption of deep learning has been its surprisingly high generalization performance (Zhang et al., 2016) .
In contrast to other machine learning techniques, continuing to add parameters to a deep network (beyond zero training loss) tends to improve generalization performance.
This is even for networks that are massively overparameterized, wherein according to traditional ML theory they should (over)fit all the training data (Neyshabur et al., 2015) .
How does training deep networks with excess capacity lead to generalization?
And how can it be that this generalization error decreases with overparameterization?
We believe that taking a functional view allows us a new, useful lens with which to explore and understand these issues.
In particular, we focus on shallow and deep fully connected univariate ReLU networks, whose parameters will always result in a Continuous Piecewise Linear (CPWL) approximation to the target function.
We provide theoretical results for shallow networks, with experiments showing that these qualitative results hold in deeper nets.
Our approach is related to previous work from (Savarese et al., 2019; Arora et al., 2019; Frankle & Carbin, 2018) in that we wish to characterize parameterization and generalization.
We differ from these other works by using small widths, rather than massively overparamaterized or infinite, and by using a functional parameterization to measure properties such as smoothness.
Other prior works such as (Serra et al., 2017; Arora et al., 2016; Montufar et al., 2014) attempt to provide theoretical upper or lower bounds to the number of induced pieces in ReLU networks, whereas we are more interested in the empirical number of pieces in example tasks.
Interestingly, (Serra et al., 2017) also takes a functional view, but is not interested in training and generalization as we are.
Previous work (Advani & Saxe, 2017) has hinted at the importance of small norm initialization, but the functional perspective allows us to prove generalization properties in shallow networks.

assistant: text: A functional approach reveals that flat initialization, preserved by gradient descent, leads to generalization ability.