Exploring The Loss Landscape Of Regularized Neural Networks Via Convex Duality

Authors: Sungyoon Kim, Aaron Mishkin, Mert Pilanci
Primary area: learning theory
Keywords: Convex duality, Machine Learning Theory, Loss Landscape, Optimal Sets

Abstract:
We discuss several aspects of the loss landscape of regularized neural networks: the structure of stationary points, connectivity of optimal solutions, path with non-increasing loss to arbitrary global optimum, and the nonuniqueness of optimal solutions, by casting the problem into an equivalent convex problem and considering its dual. Starting from two-layer neural networks with scalar output, we first characterize the solution set of the convex problem using its dual and further characterize all stationary points. With the characterization, we show that the topology of the global optima goes through a phase transition as the width of the network changes, and construct counterexamples where the problem may have a continuum of optimal solutions. Finally, we show that the solution set characterization and connectivity results can be extended to different architectures, including two layer vector-valued neural networks and parallel three-layer neural networks.

Decision: accept (oral)
Reviewer scores: 8, 8, 8, 8, 8

Reviews and discussion:
<thread>
creation date:
1730211753078

thread_type:
official_review

<thread_object>
creator: reviewer_ki4t

<content>
summary:
This manuscript proposes a characterization of the topology of the global optima in the loss landscape, focusing on regularized two-layers neural networks with free skip-connections with a partial extension to deep neural networks. The authors provide a characterization of the optimal set in terms of the width of the hidden layer, which determines a so-called "staircase of connectivity" when such a width occurs in critical values and phase transitions. The authors study the uniqueness of the minimum-norm interpolator, highlighting necessary guarantees (such as the free ski connections, bias in the training problem and unidmiensional data). An experimental study integrates the theoretical findings.

strengths:
The paper has original content, and bridges together several concepts proposed in the literature on the topic. The method of analysis is rigorous and it gives a solid contribution to the field.

weaknesses:
The manuscript presents some unclear sentences (e.g. line 198-199). There is a clear math error at line 232 (the triangle inequality holds with the reverse inequality; to be candid, I am quite sure it is a typo) and some symbols are not defined at all, not even in the Appendix (e.g. the symbol P, that occurs very often through the entire manuscript). 
There are many references to results listed in the Appendix; if relevant, I think it might be better to put them in the main manuscript.
An important reference to the characterization of loss landscapes over neural networks with regularization terms and/or skip connections is missing, also because it gives a theoretical hint on the low importance of skip connections [1].

[1] Bucarelli, M. S., D’Inverno, G. A., Bianchini, M., Scarselli, F., & Silvestri, F. (2024). A topological description of loss surfaces based on Betti Numbers. Neural Networks, 106465.

questions:
Can the authors proofread again the manuscript to eliminate typos, unclear sentences and missing notation?
Can they make the presentation of the result more readable, including relevant results from the Appendix?
Can they integrate the relevant existing literature in the Related Work section?
</content>
</thread_object>

</thread>
<thread>
creation date:
1730511350125

thread_type:
official_review

<thread_object>
creator: reviewer_eg7f

<content>
summary:
This paper studies the loss landscape of ReLU networks with $L_2$-regularization. The authors first study the canonical case of a two-layer  network with scalar output, and characterize the connectivity of the solution for different number of neurons. Then, the authors extend the results to a more general class of problems, including: minimal norm interpolation, vector-valued ReLU network, and parallel deep neural network.

strengths:
1. This paper develops a general framework to characterize the global optimum of the regularized ReLU network via convex duality. From my understanding, the key contribution of this convex duality framework in Theorem 1 is that it allows one to characterize the "direction" of the weights separately in the regularized case, which is then useful for characterizing the global optimum. I believe this contribution is novel and solid. 

2. I think the framework of characterizing the global optimal is quite general even though it is restricted to ReLU network. In particular, it do not require large over-parameterization, special scaling, or special data distributions. Thus, I believe the results can be applied to other more specific settings and is potentially useful for characterizing other properties besides the connectivity of the solutions.

weaknesses:
1.Although I believe this paper has a solid contribution, I found there's a few part I don't understand the significance:    
- I think I understand the contributions in section 3.1 and 3.2, however, I'm not sure about  In section 3.3, the authors showed that for a class of data set with dimension $=1$ that satisfies certain conditions, if the network do not have skip connection, then there are infinitely many minimal norm interpolators (which is a connected set ). I'm not sure the significance of these results, since (1) it is for a special construction of dataset. (2) it might be that those infinitely many minimal norm interpolators behave qualitatively almost the same, for example, the radius of the solution set is small. Could you discuss more on the significance of the results?

- In section 4, I understand the contribution of generalizing it to a vector-valued function. However, I'm not sure the significance of the results in Theorem 4. Since anyway you fixed all the other layers but only keep two consecutive layers, and technically I didn't see any difference from a two-layer network.  Could you discuss more on the significance of the results?

2. One main issue of the paper is the writing, especially the main part of the paper. I check the appendices, and it is much more readable. So I suggest the authors consider rearranging the content. To name a few issues\typos that confuse me when reading the main part: 

    - Line 215: and 216, what is the definition of $\mathcal{S}_i$?
    - Line 223: what the definition of "optimal model fit", what is $u_i^*, v_i^*$, and why it is unique?
    - Line 232: the triangle inequality is reversed. Also could you be more specific about the discussion between Liine 229-232?
    - The statement of Proposition 1:  First, you use $v_{i,1}$ to denote the first entry of a vector, could you specify this?  Also, you define $s_k = \sum_{i=1}^k v_{n-i+1},$ but also require $||s_k|| =1, s_n = [0,1]^\top$, could you discuss the existence of such construction? 
    - In equation (7), could you specify the dimension of the variables?

questions:
1.  A general question is about the scope of the techniques in this paper. It seems that the techniques only apply to  two-layer ReLU networks, since the problem can be equivalently written as a convex problem with regularization. It is not applicable to other activation functions and seems hard to generalize to multi-layer cases. Thus, could you elaborate more on the universality of the techniques?

2. The results in this paper require the number of neurons $m \geq m_*$. As far as  I understand, $m_*$ is the minimal number of neurons needed to achieve the optimal model. I'm wondering what would happen if $m<m_*$? Also, in general, what is the scaling of $m_*$ depending on $n,d$? 

3. The results in Theorem 2 consider the connectivity of the optimal solution set, which is equivalent to the connectivity of a path with $0$ perturbation. What about the case that allow $\epsilon$-pertubation along the path? Is the techniques still applicable?
</content>
</thread_object>

</thread>
<thread>
creation date:
1730514843119

thread_type:
official_review

<thread_object>
creator: reviewer_iep8

<content>
summary:
In this paper, the authors apply convex duality for two-layer ReLU networks to study mode connectivity and unique minimal-norm interpolator problems, while also working to generalize this framework. Specifically, the authors have:

* identified the staircase of connectivity that describes how connectivity evolves with width;
* constructed none-unique minimal-norm interpolator by breaking the uniqueness conditions;
* generalized the optimal polytope to the general cone-constrained group LASSO problem and applied it to more complicated architectures.

strengths:
The authors apply the new technique of convex duality to problems of connectivity and minimal-norm interpolation, which have been studied previously using other methods. This approach yields both generalizations of existing results and new insights into these problems. Overall, I believe this paper is a strong demonstration of how convex duality can be leveraged in the theoretical study of machine learning. The abstract concepts are clarified through figures and examples.

weaknesses:
I have some concerns with the presentation of this work. Specifically:

* If I understand correctly, the convex duality only applies to ReLU networks. This is not emphasized.
* I found Section 2 difficult to follow without prior knowledge of Pilanci & Ergen (2020). The relations between (1), (2), and (3) are mentioned but not explained (When do they have the same loss value? How do the solutions relate to each other?) Dimensions of $X$ and $y$ are not mentioned. $D_i$ is not explained.
* In Figure 1, is each red point truly a unique solution, or does it represent solutions equivalent under permutation (p-unique)? If they are p-unique solutions, readers may get the wrong impression. 
* The lower half of Figure 2 is not explained.

questions:
I wonder if the authors have any comment regarding the weaknesses.
</content>
</thread_object>

</thread>
<thread>
creation date:
1730560760945

thread_type:
official_review

<thread_object>
creator: reviewer_rcks

<content>
summary:
In this work the authors analyze multiple aspects of the loss landscape of regularized two-layer neural networks with scalar output, including the structure of stationary points, the connectivity of optimal solutions and the non uniqueness of optimal solutions. The main proof strategy is to translate the problem into an equivalent convex problem and characterize its solution set through its dual form. 
The authors show that the topology of the global optima goes through a phase transition as a function of the hidden layer width, which they term the staircase of connectivity. 
This result is extended later to networks with vector-valued outputs, and parallel deep networks of depth 3.

strengths:
- I found the "staircase of connectivity" very insightful, particularly how the connectivity properties of the optimal solutions are connected to critical widths $m^*$ and $M^*$. This finding explains how increasing the number of neurons affects the connectedness of optimal sets, and makes the observation of mode connectivity [Garipov et al. 2018] more precise. 
- The paper generalizes its findings also to vector-valued networks and deep networks with skip connection, which provides a broader framework that can be applied across different architectures.

weaknesses:
- I found the theoretical results, for instance on the staircase of connectivity, hard to interpret in practice and would benefit from more accessible explanations. While the toy example in Example 1 illustrates the concept, the absence of labels in Figure 2, as well as the notation-heavy formulation makes it difficult for readers to grasp the results intuitively. 
- Although the toy examples are helpful, the paper lacks actual empirical validation of the theoretic results. I think it would add credibility to this work, if the staircase of connectivity concept would also be tested on actual neural network architectures trained on real data. It would be interesting to see how these results scale with different data distributions and larger models. 
- Overall I found the work quite difficult to read due to the dense mathematical formalism. I also feel like the section on notations should not be in the appendix, but should - at least in a shortened version - be included in the main paper.

questions:
1. Is there a way to bound or estimate the critical widths $m^*$ and $M^*$ in practice, for instance on real datasets? 
2. In line 186: What does $h$ refer to in $\text{diag}[1 (Xh \geq 0)]$ ?
3. It is not very clear to me what lines 225-226 mean. Could you perhaps rephrase it? (That $\mathcal{P}^*_{\nu^*}$ does depend on $\nu^*$, but that the specific choice of it does not matter.)
4. Figure 2 bottom: The axis labels are missing and it is not very clear to me what the red and blue lines are supposed to represent.
5. In line 351 the author mention three interpolation problems of interest, but only discuss one problem on the minimum-norm interpolation problem. What are the other two interpolation problems and can you also extend your results to these problems?
6. The paper describes a path of nonincreasing loss that connects local to global minima. Could this insight be incorporated into practical training algorithms, such as initializing weights or guiding optimizers in large-scale training?
</content>
</thread_object>

</thread>
<thread>
creation date:
1730648358551

thread_type:
official_review

<thread_object>
creator: reviewer_ubda

<content>
summary:
The authors present a deep and novel analysis of the loss landscape and a solution in the context of regularized neural networks. They also show that the topology of the global optima undergoes a phase transition as the width of the network changes.

strengths:
The paper is well-written, easy to follow, and its results are clear and novel. The theoretical results stand out for their depth and clarity, as do the empirical results. The support of images is quite helpful when reading through some mathematical arguments or proofs of theoretical results.

weaknesses:
Perhaps a deeper discussion on the topological implications of their results would be beneficial.

questions:
No questions.
</content>
</thread_object>

</thread>
<thread>
creation date:
1732216069670

thread_type:
official_comment

<thread_object>
creator: authors

<content>
title:
General Response

comment:
We thank all reviewers for their time and thoughtful reviews. The manuscript has improved a lot thanks to the sincere reviews. 

**Presentation**

Many reviewers suggested improving the overall presentation of the paper, making it more accessible and readible. We have a revised manuscript that we believe that we have improved the presentation by:

1) Clearly defining the problem of interest and notations in the main paper
2) Making the main results (Theorem 1, 2, Proposition 3) more accessible by removing mathematically heavy notations and explaining the main concepts verbally, deferring a rigorous treatment to the appendix
3) Clarifying any undefined variables. 
4) Moving "most" relevant results from the appendix to the main paper. If the result was mathematically very complicated and is a direct extension/application of existing results (e.g. Prop F.5, Thm F.2, Prop F.1, F.2, F.3), we left the results in the appendix. 
5) We tried to clarify every unclear sentences / sections / figures (especially the second part of Figure 2) that the reviewers pointed out.

Our submission is updated with the revised version. 

**Different Activations**

Another common question was the scope of the analysis, in particular for different activation functions.

We would like to note that convex reformulations exist for many different activation functions: for example they exist for piecewise linear [1], polynomial activations [2], and threshold activations [3]. Our result are easily extended to piecewise linear activations, and similar results could be developed for different activations. Our results may not be trivially extendible to other activations such as threshold or polynomial, as their convex reformulations of  have a different form compared to ReLU. 

We sincerely appreciate the reviewers' insightful questions and thoughtful feedback. We look forward to a meaningful discussion during the discussion phase!

**References**

[1] Ergen, Tolga, and Mert Pilanci. "The Convex Landscape of Neural Networks: Characterizing Global Optima and Stationary Points via Lasso Models." arXiv preprint arXiv:2312.12657 (2023).

[2] Bartan, Burak, and Mert Pilanci. "Neural spectrahedra and semidefinite lifts: Global convex optimization of polynomial activation neural networks in fully polynomial-time." arXiv preprint arXiv:2101.02429 (2021).

[3] Ergen, Tolga, et al. "Globally optimal training of neural networks with threshold activation functions." arXiv preprint arXiv:2303.03382 (2023).
</content>
</thread_object>

</thread>


Metareview:
metareview:
This paper studies the loss landscape of regularized ReLU networks, focusing on the structure of stationary points, the connectivity of optimal solutions and the non uniqueness of optimal solutions. The authors start by considering a two-layer network with scalar output, and characterize the connectivity of the solution for different sizes of the network. Then, they consider extensions to minimal norm interpolation, vector-valued networks, and deep neural networks.

The reviewers found the results insightful (especially the 'staircase of connectivity' phenomenon) and well-integrated in the related literature. The proposed framework based on convex duality is rather general: it does not require large over-parameterization or specific scalings, which is a strong point. At the technical level, the origin of this framework is to be found in earlier work by Pilanci et al., which limits a bit the technical novelty. However, significant effort is required to tackle the settings considered here. All reviewers are positive about the paper, and so am I. Thus, I recommend it to be accepted.