# Learning Hyperbolic Representations of Topological Features

international conference on learning representations, 2020.

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Abstract:

Learning task-specific representations of persistence diagrams is an important problem in topological data analysis and machine learning. However, current state of the art methods are restricted in terms of their expressivity as they are focused on Euclidean representations. Persistence diagrams often contain features of infinite persiste...More

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Introduction

- Persistent homology is a topological data analysis tool which tracks how topological features appear and disappear as the authors analyze the data at different scales or in nested sequences of subspaces [1; 2].
- Under some assumptions, one can approximately reconstruct the input space from a diagram [4]
- Despite their strengths, the space of persistence diagrams lacks structure as basic operations, such as addition and scalar multiplication, are not defined.
- A filtration of K is a nested sequence of subcomplexes that starts with the empty complex and ends with K,

Highlights

- Persistent homology is a topological data analysis tool which tracks how topological features appear and disappear as we analyze the data at different scales or in nested sequences of subspaces [1; 2]
- The lifespan of the feature is called persistence. Persistent homology summarizes these topological characteristics in a form of multiset called persistence diagram, which are highly robust and versatile descriptors of the data
- We focus on persistence diagrams extracted from graphs and grey-scale images
- To the best of our knowledge, method for learning representations of persistence diagrams in the Poincare ball
- The main benefit of using the Poincare space is that by allowing the representations of essential features to get infinitesimally close to the boundary of the ball their distance to non-essential features approaches infinity, preserves their relative importance
- Directions for future work include the learning of filtration and/or scale end-to-end as well as to investigate whether or not there exists some hyperbolic trend in distances appearing in persistence diagrams that justify the improved performance especially on small graphs

Methods

- The authors focus on persistence diagrams extracted from graphs and grey-scale images.
- In both cases, the learning task is classification and the authors compare the performance of the method against other methods.
- The authors' representation acts as an input to a neural network architecture and the parameters are learned end-to-end via standard gradient methods.
- The authors implemented all algorithms in TensorFlow 2.2 using the TDA-Toolkit and the Scikit-TDA3 for extracting persistence diagrams and run all experiments on the Google Cloud AI Platform.
- The authors provide the code to reproduce the results in the supplementary material

Results

- Let D, E be two persistence diagrams and η : D → E bijection that achieves the infinum in Eq..
- Consider the subset D0 = D \ R∆, which is essentially the original diagram without the points in the diagonal.
- The authors have the following sequence of inequalities dB(Φ(D, θ), Φ(E, θ)) [21] = dB expx0.
- Logx0 φ(ρ(x)) , expx0 logx0 φ(ρ(x)) [22] x∈E.
- ≤ Ke logx0 (φ(ρ(x))) − logx0 (φ(ρ(x)))) [23]

Conclusion

- To the best of the knowledge, method for learning representations of persistence diagrams in the Poincare ball.
- The authors' main motivation for introducing such method is that persistence diagrams often contain topological features of infinite persistence the representational capacity of which may be bottlenecked when representing them in Euclidean spaces.
- This stems from the fact that Euclidean spaces cannot assign infinite distance to finite points.
- Directions for future work include the learning of filtration and/or scale end-to-end as well as to investigate whether or not there exists some hyperbolic trend in distances appearing in persistence diagrams that justify the improved performance especially on small graphs

Summary

## Introduction:

Persistent homology is a topological data analysis tool which tracks how topological features appear and disappear as the authors analyze the data at different scales or in nested sequences of subspaces [1; 2].- Under some assumptions, one can approximately reconstruct the input space from a diagram [4]
- Despite their strengths, the space of persistence diagrams lacks structure as basic operations, such as addition and scalar multiplication, are not defined.
- A filtration of K is a nested sequence of subcomplexes that starts with the empty complex and ends with K,
## Methods:

The authors focus on persistence diagrams extracted from graphs and grey-scale images.- In both cases, the learning task is classification and the authors compare the performance of the method against other methods.
- The authors' representation acts as an input to a neural network architecture and the parameters are learned end-to-end via standard gradient methods.
- The authors implemented all algorithms in TensorFlow 2.2 using the TDA-Toolkit and the Scikit-TDA3 for extracting persistence diagrams and run all experiments on the Google Cloud AI Platform.
- The authors provide the code to reproduce the results in the supplementary material
## Results:

Let D, E be two persistence diagrams and η : D → E bijection that achieves the infinum in Eq..- Consider the subset D0 = D \ R∆, which is essentially the original diagram without the points in the diagonal.
- The authors have the following sequence of inequalities dB(Φ(D, θ), Φ(E, θ)) [21] = dB expx0.
- Logx0 φ(ρ(x)) , expx0 logx0 φ(ρ(x)) [22] x∈E.
- ≤ Ke logx0 (φ(ρ(x))) − logx0 (φ(ρ(x)))) [23]
## Conclusion:

To the best of the knowledge, method for learning representations of persistence diagrams in the Poincare ball.- The authors' main motivation for introducing such method is that persistence diagrams often contain topological features of infinite persistence the representational capacity of which may be bottlenecked when representing them in Euclidean spaces.
- This stems from the fact that Euclidean spaces cannot assign infinite distance to finite points.
- Directions for future work include the learning of filtration and/or scale end-to-end as well as to investigate whether or not there exists some hyperbolic trend in distances appearing in persistence diagrams that justify the improved performance especially on small graphs

- Table1: Classification accuracy (mean±std or min-max range, if available)
- Table2: Classification accuracy

Related work

- To address these issues, several vectorization methods have been proposed. Some of the earliest approaches are based on kernels, i.e., generalized products that turn persistence diagrams into elements of a Hilbert space. Kusano et al [5] propose a persistence weighted Gaussian kernel which allows them to explicitly control the effect of persistence. Alternatively, Carrière et al [6] leverage the sliced Wasserstein distance to define a kernel that mimics the distance between diagrams. The approaches by Bubenik [7] based on persistent landscapes, by Reininghaus et al [8] based on scale space theory and by Le et al [9] based on the Fisher information metric are along the same line of work. The major drawback in utilizing kernel methods is that they suffer from scalability issues as the training scales poorly with the number of samples.

Study subjects and analysis

samples: 1000

We evaluate our approach using social graphs from [26]. The REDDIT-BINARY dataset contains 1000 samples and the graphs correspond to online discussion threads. The task is to identify to which community a given graph belongs (question/answer-based community or a discussion-based community)

Reference

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