[论文精读]Semi-Supervised Classification with Graph Convolutional Networks

论文原文：[1609.02907] Semi-Supervised Classification with Graph Convolutional Networks (arxiv.org)

论文代码：GitHub - tkipf/gcn: Implementation of Graph Convolutional Networks in TensorFlow

英文是纯手打的！论文原文的summarizing and paraphrasing。可能会出现难以避免的拼写错误和语法错误，若有发现欢迎评论指正！文章偏向于笔记，谨慎食用！

目录

1. 省流版

1.1. 心得

1.2. 论文框架图

2. 论文逐段阅读

2.1. Abstract

2.2. Introduction

2.3. Fast approximate convolutions on graphs

2.3.1. Spectral graph convolutions

2.3.2. Layer-wise linear model

2.4. Semi-supervised node classification

2.4.1. Example

2.4.2. Implementation

2.5. Related work

2.5.1. Graph-based semi-supervised learning

2.5.2. Neural networks on graphs

2.6. Experiments

2.6.1. Datasets

2.6.2. Experimental set-up

2.6.3. Baselines

2.7. Results

2.7.1. Semi-supervised node classifiication

2.7.2. Evaluation of propagation model

2.7.3. Training time per epoch

2.8. Discussion

2.8.1. Semi-supervised model

2.8.2. Limitations and future work

2.9. Conclusion

3. 知识补充

3.1. Skip-gram

3.2. Bag of words

3.3. Spectral graph convolutions

4. Reference List

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1. 省流版

1.1. 心得

（1）怎么开头我就不知道在说什么啊这个论文感觉表述不是很清晰？

（2）数学部分推理很清晰

（3）要是模型展示更清晰就好了

1.2. 论文框架图

2. 论文逐段阅读

2.1. Abstract

        ①Their convolution is based on localized first-order approximation

        ②They encode node features and local graph structure in hidden layers

2.2. Introduction

        ①The authors think adopting Laplacian regularization in the loss function helps to label:

where  represents supervised loss with labeled data, 

 is a differentiable function,

 denotes weight,

 denotes matrix with combination of node feature vectors,

 represents the unnormalized graph Laplacian,

 is adjacency matrix,

 is degree matrix.

        ②The model trains labeled nodes and is able to learn labeled and unlabeled nodes

        ③GCN achieves higher accuracy and efficiency than others

2.3. Fast approximate convolutions on graphs

        ①GCN (undirected graph):

where  denotes self-connection adjacency matrix, which means ,

 denotes identity matrix,

 denotes self-connection degree matrix,

 represents the trainable weight matrix in -th layer,

 denotes the activation matrix in -th layer,

 represents activation function

2.3.1. Spectral graph convolutions

        ①Spectral convolutions on graphs:

where the filter ,

 comes from normalized graph Laplacian and is the matrix of 's eigenvectors,

 denotes a diagonal matrix with eigenvalues.

        ②However, it is too time-consuming to compute matrix especially for large graph. Ergo, approximating it in -th order by Chebyshev polynomials:

where ,

 denotes Chebyshev coefficients vector,

recursive Chebyshev polynomials are  with baseline  and 

        ③Then get new function:

where , .

        ④Through this approximation method, time complexity reduced from  to 

2.3.2. Layer-wise linear model

        ①Then, the authors stack the function above to build multiple conv layers and set , 

        ②They simplified 2.3.1. ③ to:

where  and  are free parameters 

        ③Nevertheless, more parameters bring more overfitting problem. It leads the authors change the expression to:

where they define  ,

eigenvalues are in  .

But keep using it may cause exploding/vanishing gradients or numerical instabilities.

        ④Then they adjust 

        ⑤The convolved signal matrix :

where  denotes input channels, namely feature dimensionality of each node,

 denotes the number of filters or feature maps,

 represents matrix of filter parameters

2.4. Semi-supervised node classification

        The whole model shows:

2.4.1. Example

        ①They take 2 layers model

        ②Forward propagation:

where  is a weight matrix with  feature maps, it crosses input layer to hidden layer,

 is a weight matrix crossing hidden layer to output layer,

ReLU is for every row

        ③Cross entropy:

where  is the set of node indices of which nodes with labels

        ④They adopt mini-batch stochastic gradient descent

2.4.2. Implementation

        ①Framework: TensorFlow

        ②Method: sparse-dense matrix multiplications

        ③Computational complexity: 

2.5. Related work

2.5.1. Graph-based semi-supervised learning

（1）Previous graph representations:

        ①Explicit graph Laplacian regularization: such as deep semi-supervised embedding, manifold regularization, label propagation 

        ②Graph-embedding

（2）Recent works:

        ①DeepWalk

        ②LINE

        ③node2vec

2.5.2. Neural networks on graphs

        ①Briefly introduced other works

        ②Their approach is based on spectral graph convolutional neural networks

2.6. Experiments

They tested their model in:

        ①Semi-supervised document classification in citation networks

        ②Semi-supervised entity classification in a bipartite graph extracted from a knowledge graph

        ③Evaluation of various graph propagation models

        ④Run-time analysis on random graphs

bipartite  adj.两部分组成的;有两个部分的

2.6.1. Datasets

        ①Datasets table:

where documents represented by nodes, citation links represented by edges;

also, ;

NELL is a bipartite graph dataset including 55,864 relation nodes and 9,891 entity nodes

        ②Citation networks: sparse bag-of-words feature vectors and list of citation links for and between documents are contained. Besides, undirected edges are citation links and 20 labels for one class only

        ③NELL: edges are labeled, directed connections. Additionally, one-hot representation is adopted in each node

        ④Random graphs: randomly genetate graph with N nodes and 2N edges

2.6.2. Experimental set-up

        ①Labeled sample test set: 1,000

        ②They usually adopt 2 layers, but further test extra 10 layers in appendix b

        ③Epoch: under 200

        ④Optimizer: Adam

        ⑤Stop training when there is no decreasing of loss for 10 consecutive epochs

        ⑥Hidden layer includes 32 units

        ⑦Omit dropout and regularization

2.6.3. Baselines

        ①They train local classifier with labeled data and then train unlabeled with a random node ordering for 10 iterations

        ②L2 regularization parameter and aggregation operator selected by performance of validation set

2.7. Results

2.7.1. Semi-supervised node classifiication

（1）Comparison of accuracy table with mean accuracy of 100 times in random ordering:

（2）Parameters for Citeseer, Cora and Pubmed

        ①Dropout rate: 0.5

        ②L2 regularization: 5e-4

        ③Hidden units number: 16

（3）Parameters for NELL

        ①Dropout rate: 0.1

        ②L2 regularization: 1e-5

        ③Hidden units number: 64

2.7.2. Evaluation of propagation model

        They compared variants of their propagation model on citation network datasets, where GCN is the bold one:

2.7.3. Training time per epoch

        Mean training time per epoch of 100 epochs. Each epoch contains forward propagation, cross entropy computing, backward propagation:

where * denotes memory explosion

2.8. Discussion

2.8.1. Semi-supervised model

        ①The authors attribute low efficiency of graph-Laplacian regularization to only encoding similar nodes

        ②In addition, they think skip-gram is hard to optimize

        ③Then, they reckon renormalized propagation is better than na¨ıve 1 st-order model and higher-order graph convolutional models with Chebyshev polynomials

2.8.2. Limitations and future work

        ①Lack of memory. Slightly alleviated by mini-batch stochastic gradient descent

        ②The model does not support directed edge or edge features. （后面说的那句话不是很能理解，翻译成中文是“通过将原始有向图表示为具有原始图中表示边缘的附加节点的无向二部图，可以同时处理有向边和边缘特征”。难道说原本是一个普通的图，把每条边都变成一个节点放在另一边，然后和原先的节点合起来变成二部图？但是很奇怪啊）

        ③Weight of self-connection might vary in different graphs:

2.9. Conclusion

        They proposed a novel and efficient semi-supervised graph convolutional network

3. 知识补充

3.1. Skip-gram

相关链接：理解 Word2Vec 之 Skip-Gram 模型 - 知乎 (zhihu.com)

3.2. Bag of words

相关链接：使用 Python 的 NLP 中的单词袋简介 |什么是 BoW？ (mygreatlearning.com)

3.3. Spectral graph convolutions

相关链接：谱图卷积 - 知乎 (zhihu.com)

4. Reference List

Kipf, T. & Welling, M. (2017) 'Semi-Supervised Classification with Graph Convolutional Networks', ICLR 2017, doi: https://doi.org/10.48550/arXiv.1609.02907