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Title: Matching Distributions via  Optimal Transport for Semi-Supervised Learning

Paper content:
Semi-Supervised Learning (SSL) approaches have been an influential framework for the usage of unlabeled data when there is not a sufficient amount of labeled data available over the course of training.
SSL methods based on Convolutional Neural Networks (CNNs) have recently provided successful results on standard benchmark tasks such as image classification.
In this work, we consider the general setting of  SSL problem where the labeled and unlabeled data  come from the same underlying probability distribution.
We  propose a new approach that adopts  an Optimal Transport (OT) technique serving as a metric of similarity between discrete empirical probability measures to  provide pseudo-labels for the unlabeled data, which can then be used in conjunction with the initial labeled data to train the CNN model in an SSL manner.
We have evaluated and compared our proposed method with state-of-the-art SSL algorithms on standard datasets to demonstrate the superiority and effectiveness of our  SSL algorithm.
Recent developments in CNNs have provided promising results for many applications in machine learning and computer vision Krizhevsky et al. (2012) ; Zagoruyko & Komodakis (2016) .
However, the success of CNN models requires a vast amount of well-annotated training data, which is not always feasible to perform manually Krizhevsky et al. (2012) .
There are essentially two different solutions that are usually used to deal with this problem:
1) Transfer Learning (TL) and
2) SemiSupervised Learning (SSL).
In TL methods Tan et al. (2018) , the learning of a new task is improved by transferring knowledge from a related task which has already been learned.
SSL methods Oliver et al. (2018) , however, tend to learn discriminative models that can make use of the information from an input distribution that is given by a large amount of unlabeled data.
To make use of unlabeled data, it is presumed that the underlying distribution of data has some structure.
SSL algorithms make use of at least one of the following structural assumptions: continuity, cluster, or manifold Chapelle et al. (2009) .
In the continuity assumption, data which are close to each other are more likely to belong to the same class.
In the cluster assumption, data tends to form discrete clusters, and data in the same cluster are more likely to share the same label.
In the manifold assumption, data lies approximately on a manifold of much lower dimension than the input space which can be classified by using distances and densities defined on the manifold.
Thus, to define a natural similarity distance or divergence between probability measures on a manifold, it is important to consider the geometrical structures of the metric space in which the manifold exists Bronstein et al. (2017) .
There are two principal directions that model geometrical structures underlying the manifold on which the discrete probability measures lie.
The first direction is based on the principal of invariance, which relies on the criterion that the geometry between probability measures should be invariant under invertible transformations of random variables.
This perspective is the foundation of the theory of information geometry, which operates as a base for the statistical inference Amari (2016) .
The second direction is established by the theory of Optimal Transport (OT), which exploits prior geometric knowledge on the base space in which random variables are valued Villani (2008) .
Computing OT or Wasserstein distance between two random variables equals to achieving a coupling between these two variables that is optimal in the sense that the expectation of the transportation cost between the first and second variables is minimal.
The Wasserstein distance between two probability measures considers the metric properties of the base space on which a structure or a pattern is defined.
However, traditional information-theoretic divergences such as the Hellinger divergence and the Kullback-Leibler (KL) divergence are not able to properly capture the geometry of the base space.
Thus, the Wasserstein distance is useful for the applications where the structure or geometry of the base space plays a significant role Amari & Nagaoka (2007) .
In this work, similar to other SSL methods, we make a structural assumption about the data in which the data are represented by a CNN model.
Inspired by the Wasserstein distance, which exploits properly the geometry of the base space to provide a natural notion of similarity between the discrete empirical measures, we use it to provide pseudo-labels for the unlabeled data to train a CNN model in an SSL fashion.
Specifically, in our SSL method, labeled data belonging to each class is a discrete measure.
Thus, all the labeled data create a measure of measures and similarly, the pool of unlabeled data is also a measure of measures constructed by data belonging to different classes.
Thus, we design a measure of measures OT plan serving as a similarity metric between discrete empirical measures to map the unlabeled measures to the labeled measures based on which, the pseudo-labels for the unlabeled data are inferred.
Our SSL method is based on the role of Wasserstein distances in the hierarchical modeling Nguyen et al. (2016) .
It stems from the fact that the labeled and unlabeled datasets hierarchically create a measure of measures in which each measure is constructed by the data belonging to the same class.
Computing the exact Wasserstein distance, however, is computationally expensive and usually is solved by a linear program (Appendix A and D ).
Cuturi (2013) introduced an interesting method which relaxes the OT problem using the entropy of the solution as a strong convex regularizer.
The entropic regularization provides two main advantageous: 1) The regularized OT problem relies on Sinkhorns algorithm Sinkhorn (1964) that is faster by several orders of magnitude than the exact solution of the linear program.
2) In contrast to exact OT, the regularized OT is a differentiable function of their inputs, even when the OT problem is used for discrete measures.
These advantages have caused that the regularized OT to receive a lot of attention in machine learning applications such as generating data ; Gulrajani et al. (2017) , designing loss function Frogner et al. (2015) , domain adaptation Damodaran et al. (2018) ; Courty et al. (2017) , clustering Cuturi & Doucet (2014) ; Mi et al. (2018) and low-rank approximation Seguy & Cuturi (2015) .
We proposed a new SSL method based on the optimal transportation technique in which unlabeled data masses are transported to a set of labeled data masses, each of which is constructed by data belonging to the same class.
In this method, we found a mapping between the labeled and unlabeled masses which was used to infer pseudo-labels for the unlabeled data so that we could use them to train our CNN model.
Finally, we experimentally evaluated our SSL method to indicate its potential and effectiveness for leveraging the unlabeled data when labels are limited during the training.

assistant: text: We propose a new algorithm based on the optimal transport to train a CNN in an SSL fashion.