Graph View-Consistent Learning Network

1 INTRODUCTION . Convolutional neural networks ( CNNs ) ( Krizhevsky et al. , 2012 ) performed outstandingly in solving problems such as image classification ( Rawat & Wang , 2017 ) , semantic segmentation ( Kampffmeyer et al. , 2016 ) and machine translation ( Cho et al. , 2014 ) etc . This is because CNNs can effectively reuse the convolution kernel and use the given input to train optimal parameters . The original data mentioned in above problems all have a grid-like data structure , that is , Euclidean spatial data . In reality , there are also lots of non-Euclidean spatial data , such as social networks , telecommunication networks , biological networks , and brain connection structures , etc . These data are usually represented in the form of graphs , where every node in the graph represents a single individual . Graph problems can be roughly divided into there direction : link prediction ( Zhang & Chen , 2018 ) , graph classification ( Zhang et al. , 2018a ) and node classification ( Kipf & Welling , 2016 ) . In this paper , we focus on semi-supervised node classification when the label rate is very low . Many new methods have been proposed to generalize the convolution operation to process graph structure data on arbitrary graphs for node classification . These methods can be divided into spatial convolution and spectral convolution methods ( Zhang et al. , 2018b ) . For spatial methods , they directly define graph convolution by designing certain operations on node ’ s neighbors . For example , Duvenaud et al . ( 2015 ) propose a convolutional neural network that can directly operate on graph data , which can provide an end-to-end feature learning method ; Atwood & Towsley ( 2016 ) propose a fusion convolutional neural network ( DCNNS ) , which introduces the graph fusion method to incorporate the context information of the nodes in the graph node classification ; The Graph Attention Network ( GATs ) ( Veličković et al. , 2017 ) introduces the attention mechanism into the graph data processing to construct the attention layer for semi-supervised learning . The spectral method generally defines the graph convolution operation on spectral representation of graph . For example , Bruna et al . ( 2013 ) propose that graph convolution can be defined in the Fourier domain based on the eigenvalue decomposition of the graph Laplacian matrix ; Defferrard et al . ( 2016 ) propose to use the Chebyshev expansion of the graph Laplacian to approximate the spectral domain filtering , which can avoid high computational complexity brought by eigenvalue decomposition ; Kipf & Welling ( 2016 ) propose a simpler Graph Convolutional Networks ( GCNs ) for semi-supervised learning , which can achieve higher classification accuracy by using a simple two-layer networks . However , large training and validation sets are required in these methods to complish effect classification task , while obtaining true labels is time-consuming , laborious and costly . And they send original graph node features directly into networks for training , however , there are lots of redundant information in the original features of the nodes . In order to train an efficient model with only a few label nodes and even without validation , we put forward our own method : graph view-consistent learning network ( GVCLN ) , which constructs a node classification network based on the consistency between two views . First , we independently train two-view encoders ( can be different ) to obtain two representations of every node . The function of the viewers is converting high-dimensional node features into low-dimensional embeddings ( Zhu et al. , 2020 ) . The clustering hypothesis ( Vandenberg & Matthias , 1977 ) show that examples in the same cluster are more likely to have the same label . According to this hypothesis , the decision boundary should try to pass through the place where the data is relatively sparse , so as to avoid dividing the data points in dense clusters on both sides of the decision boundary . Although the two views have different viewing angles , their observation objects are the same , so their observation results should be consistent . Therefore , the features encoded by the two viewers should make the decision boundary pass through the place where the date is sparse , that is , there should be consistency between the two views . Then , we design three loss functions , namely , supervised loss function , consistency loss function , and pseudo-label loss function . The supervised loss uses the known labeled set , while a view-consistent loss is applied to the two views to obtain the consistent representation and a pseudo-label loss is designed by using the common high-confidence predictions as pseudo label . GVCLN with these loss functions can obtain the view-consistent representation of the original feature . Our contributions are summarized as follows : • We propose a graph view-consistent learning framework for semi-supervised node classification , which fully demonstrates the theoretical structure of graph view-consistency . • We design GVCLN to successfully tackle label insufficiency in semi-supervised learning . • We demonstrate the high efficacy and efficiency of the proposed methods on various semisupervised node classification tasks . 2 RELATED WORK . 2.1 NOTATIONS . A graph contains two parts : node and edge . Each node represents an individual , which can be a paper or a person , etc . The edge indicates a connection between two nodes . If the edge connecting two nodes in the graph is directional , it is directed graph , otherwise it is undirected graph . A simple and connected undirected graph can be written as G = ( V , E ) , where V is the node set and E is the set of edges . n = |V | represents the number of all nodes in G. Considering that the node itself has a great influence on the graph structure , the graph used in the calculation of the network is generally a self-loop graph , namely G̃ = ( V , Ẽ ) , which attaches a self-loop to each node in G. A denotes the adjacency matrix andD is the diagonal degree matrix . Therefore , the adjacency matrix and diagonal degree matrix of G̃ are defined as Ã = A + I and D̃ , respectively . I indicates the identity matrix . The node feature matrix is X ∈ Rn×d , in which , each node i is associated with a d-dimensional feature vector xi . The normalized graph Laplacian matrix is defined as L = I − D−1/2AD−1/2 , which is a symmetric positive semidefinite matrix with eigendecomposition UΛU > , where Λ is a diagonal matrix of the eigenvalues of L , and U ∈ Rn×n is a unitary matrix that consists of the eigenvectors of L. The graph convolution operation between signal x and filter gγ ( Λ ) = diag ( γ ) is defined as gγ ( L ) ∗ x = Ugγ ( Λ ) U > x , where the parameter γ ∈ Rn corresponds to a vector of spectral filter coefficients . 2.2 FEATURE AGGREGATION . Graph neural Networks ( GNNs ) follow a neighborhood feature aggregation scheme . Many GNNs use different aggregation approaches . Here , we introduce two most classic feature aggregation ways . Graph convolutional network ( GCN ) ( Kipf & Welling , 2016 ) has outstanding effects in solving the problem of semi-supervised node classification . Kipf & Welling ( 2016 ) define the graph convolutional layer for feature aggregation in GCN . Symmetrically normalized operation is used on the adjacency matrix of G̃ , i.e. , Â = D̃−1/2ÃD̃−1/2 . The graph convolutional layer is defined by , H l+1 = σ ( ÂH lΘ ) , ( 1 ) where H l is the feature of the l-th layer , Θ is the trainable weight matrix of the layer , and σ ( · ) is a nonlinear activation function , e.g. , ReLU ( · ) = max ( 0 , · ) . Veličković et al . ( 2017 ) present graph attention network ( GAT ) , novel neural network architectures that operate on graph-structured data , leveraging masked self-attentional layers to address the shortcomings of prior methods based on graph convolutions or their approximations . By giving different attention weights αij to different neighbor nodes , each node-i are able to aggregate their neighborhoods features to update their own features . The graph attention layer is defined by , hl+1i = σ ( ∑ j∈Ni αijWh l j ) , ( 2 ) where hi represents latent embedding feature at node i , W is a linear transformations weight matrix , and αij are normalized attention coefficients . 3 MODEL ARCHITECTURE . In this section , we first introduce the overall abstract structure of GVCLN , and then use graph convolutional layer and graph attention layer to build a specific GVCLN model to achieve viewconsistent learning for node classification . 3.1 MODEL ARCHITECTURE OVERVIEW . A general understanding of our GVCLN is shown in Fig . 1 . In GVCLN , feature X and the graph A are encoded by two different viewers at the same time , namely Viewer 1 and Viewer 2 , the two viewers aggregate features with three-head graph layers . Viewer 1 uses the graph convolution layer and Viewer 2 is the graph attention layer . The three-head representations , H ( v ) 1 , H ( v ) 2 , H ( v ) 3 , v ∈ { 1 , 2 } , are of concatenation and perform dropout . After dropout , the two-view latent features , Z ( 1 ) and Z ( 2 ) , pass through a same non-linear graph convolutional layer to obtain the prediction , P ( 1 ) and P ( 2 ) . A view-consistency loss function is made for the dual-view output consistently . 3.2 VIEW-CONSISTENCY LEARNING . We consider applying view-consistent learning to the view 1 and view 2 in Figure 1 . Inspired by contrastive learning , Chen et al . ( 2020 ) inputs the original sample and the sample after data augmentation ( flipping or cropping , etc . ) into two same views respectively . Since data augmentation does not change the label information of sample , the output representations of two views are similar . By maximizing the consistency of the two views , better results can be achieved . In this paper , we do not do data augmentation to the data , but used two different viewers . We believe that the same data has the similar output through different viewers and classifiers . In Figure 1 , there is a consistency loss function between the view 1 and the view 2 to realize view-consistent learning of two views . The soft cross entropy function is employed to construct the consistency loss function : ` view = − 1 n n∑ i=1 k∑ j=1 p ( 1 ) ij ln p ( 2 ) ij . ( 3 ) where k is the number of class , P ( 1 ) = [ p ( 1 ) ij ] and P ( 2 ) = [ p ( 2 ) ij ] are the predictions of the two viewers for node-i , respectively . The concept of co-training was proposed by ( Blum & Mitchell , 1998 ) , co-training assumes that each data point has two views , and each view is sufficient for learning an effective model . Co-training learns separately on two views with labeled data samples to obtain two different learners and then uses the unlabeled data samples in the respective views of the two learners to make predictions . The basic learning process of collaborative training is : Each learner selects the data sample with the top high-confidence predictions in unlabeled data and adds it to the labeled data sample set of another learner for training . This process is repeated until a specific stopping condition is met . Co-training needs to meet the following two conditions : • Views are sufficient and redundant , that is , for each view , if enough labeled data is given , each view can learn a learner with good performance . • Conditionally independent , that is , the label information of each view is conditionally independent of the label information of another view . Our view-consistent learning method is similar to co-training , which requires pseudo-labels for training . The selected pseudo-label data can not be completely clean ( the predicted label is exactly the same as the real label ) , but contains noise ( the situation where the predicted label is inconsistent with the real label ) . Due to the powerful representation capabilities of deep neural networks , even noisy labels can be fitted by deep neural networks . Based on ( Han et al. , 2018 ) , which can train deep neural networks with extremely noisy labels , we propose our own training strategy . We use labeled samples to train two independent learners , namely view 1 and view 2 . When the two-view networks are well trained , according to their predictions , the predicted labels of some nodes with the same prediction representations and common high-confidence prediction are selected as pseudo labels . Then , pseudo label set is added to the original labeled set for training . This is repeated until the stop condition is reached . We believe that pseudo labels have similar importance as real labels , but the number of pseudo-labels taken each time can be controlled by the loss function value and the accuracy of the verification set , and different numbers of pseudo-labels can be given to data sets with different label rates . GVCLN selects nodes with high confidence and with same prediction results in the two views as pseudo labels . The pseudo labels are voted with high-confidence predictions of the two viewers taken from unlabeled nodes . The pseudo-label loss has the same effect as consistency loss , that is : making the two views more and more consistent . Meanwhile , supervised loss has the function of fine-tuning and controlling the direction of network optimization . We choose cross entropy as a loss function to calculate the loss for both the labeled samples and the pseudo labels : CEi = − k∑ j=1 yij ln pij , ( 4 ) where yij denotes the ground-truth label , i.e. , the i-th node belongs to the j-th class , pij is the network prediction probability of the i-th label . Thus , the supervised loss function of labeled samples is : ` sup = − 1 s s∑ i=1 k∑ j=1 yij ln pij . ( 5 ) where s denotes the number of labeled nodes , For pseudo labels , we also use cross entropy to calculate the loss function : ` pseudo = − 1 t t∑ m=1 k∑ i=1 y′ij ln pij . ( 6 ) where y′ij is the pseudo label and t denote the number of nodes of the pseudo labels .

Summary: In this paper, a graph view-consistent learning framework (GVCLN) is proposed. Specifically, two view learners are used to give predictions for the input. Then, a consistency loss is employed to force the two viewers giving the same predictions. Moreover, a co-training scheme is proposed to alleviate the label sparsity problem.