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Title: Neural Subgraph Isomorphism Counting

Paper content:
In this paper, we study a new graph learning problem: learning to count subgraph isomorphisms.
Although the learning based approach is inexact, we are able to generalize to count large patterns and data graphs in polynomial time compared to the exponential time of the original NP-complete problem.
Different from other traditional graph learning problems such as node classification and link prediction, subgraph isomorphism counting requires more global inference to oversee the whole graph.
To tackle this problem, we propose a dynamic intermedium attention memory network (DIAMNet) which augments different representation learning architectures and iteratively attends pattern and target data graphs to memorize different subgraph isomorphisms for the global counting.
We develop both small graphs (<= 1,024 subgraph isomorphisms in each) and large graphs (<= 4,096 subgraph isomorphisms in each) sets to evaluate different models.
Experimental results show that learning based subgraph isomorphism counting can help reduce the time complexity with acceptable accuracy.
Our DIAMNet can further improve existing representation learning models for this more global problem.
Graphs are general data structures widely used in many applications, including social network analysis, molecular structure analysis, natural language processing and knowledge graph modeling, etc.
Learning with graphs has recently drawn much attention as neural network approaches to representation learning have been proven to be effective for complex data structures (Niepert et al., 2016; Kipf & Welling, 2017; Hamilton et al., 2017b; Schlichtkrull et al., 2018; Velickovic et al., 2018; Xu et al., 2019) .
Most of existing graph representation learning algorithms focus on problems such as node classification, linking prediction, community detection, etc. (Hamilton et al., 2017a) .
These applications are of more local decisions for which a learning algorithm can usually make inferences by inspecting the local structure of a graph.
For example, for the node classification problem, after several levels of neighborhood aggregation, the node representation may be able to incorporate sufficient higher-order neighborhood information to discriminate different classes (Xu et al., 2019) .
In this paper, we study a more global learning problem: learning to count subgraph isomorphisms (counting examples are shown as Figure 1 ).
Although subgraph isomorphism is the key to solve graph representation learning based applications (Xu et al., 2019) , tasks of identifying or counting subgraph isomorphisms themselves are also significant and may support broad applications, such as bioinformatics (Milo et al., 2002; Alon et al., 2008) , chemoinformatics (Huan et al., 2003) , and online social network analysis (Kuramochi & Karypis, 2004) .
For example, in a social network, we can solve search queries like "groups of people who like X and visited Y-city/state."
In a knowledge graph, we can answer questions like "how many languages are there in Africa speaking by people living near the banks of the Nile River?"
Many pattern mining algorithms or graph database indexing based approaches have been proposed to tackle subgraph isomorphism problems (Ullmann, 1976; Cordella et al., 2004; He & Singh, 2008; Han et al., 2013; Carletti et al., 2018) .
However, these approaches cannot be applied to large-scale graphs because of the exponential time complexity.
Thanks to the powerful graph representation learning models which can effectively capture local structural information, we can use a learning algorithm to learn how to count subgraph isomorphisms from a lot of examples.
Then the algorithm can scan a large graph and memorize all necessary local information based on a query pattern graph.
In this case, although learning based approaches can be inexact, we can roughly estimate the range of the number of subgraph isomorphism.
This can already help many applications that do not require exact match or need a more efficient pre- processing step.
To this end, in addition to trying different representation learning architectures, we develop a dynamic intermedium attention memory network (DIAMNet) to iteratively attend the query pattern and the target data graph to memorize different local subgraph isomorphisms for global counting.
To evaluate the learning effectiveness and efficiency, we develop a small (≤ 1,024 subgraph isomorphisms in each graph) and a large (≤ 4,096 subgraph isomorphisms in each graph) dataset and evaluate different neural network architectures.
Our main contributions are as follows.
• To our best knowledge, this is the first work to model the subgraph isomorphism counting problem as a learning problem, for which both the training and prediction time complexities are polynomial.
• We exploit the representation power of different deep neural network architectures in an end-toend learning framework.
In particular, we provide universal encoding methods for both sequence models and graph models, and upon them we introduce a dynamic intermedium attention memory network to address the more global inference problem for counting.
• We conduct extensive experiments on developed datasets which demonstrate that our framework can achieve good results on both relatively large graphs and large patterns compared to existing studies.
In this paper, we study the challenging subgraph isomorphism counting problem.
With the help of deep graph representation learning, we are able to convert the NP-complete problem to a learning based problem.
Then we can use the learned model to predict the subgraph isomorphism counts in polynomial time.
Counting problem is more related to a global inference rather than only learning node or edge representations.
Therefore, we have developed a dynamic intermedium attention memory network to memorize local information and summarize for the global output.
We build two datasets to evaluate different representation learning models and global inference models.
Results show that learning based method is a promising direction for subgraph isomorphism detection and counting and memory networks indeed help the global inference.
We also performed detailed analysis of model behaviors for different pattern and graph sizes and labels.
Results show that there is much space to improve when the vertex label size is large.
Moreover, we have seen the potential real-world applications of subgraph isomorphism counting problems such as question answering and information retrieval.
It would be very interesting to see the domain adaptation power of our developed pretrained models on more real-world applications.
As shown in Figure 7 , different interaction modules perform differently in different views.
We can find MaxPool always predicts higher counting values when the pattern is small and the graph is large, while AttnPool always predicts very small numbers except when the pattern vertex size is 8, and the graph vertex size is 64.
The same result appears when we use edge sizes as the x-axis.
This observation shows that AttnPool has difficulties predicting counting values when either of the pattern and the graph is small.
It shows that attention focuses more on the zero vector we added rather than the pattern pooling result.
Our DIAMNet, however, performs the best in all pattern/graph sizes.
When the bins are ordered by vertex label sizes or edge label sizes, the performance of all the three interaction modules among the distribution are similar.
When bins are ordered by vertex label sizes, we have the same discovery that AttnPool prefers to predict zeros when then patterns are small.
MaxPool fails when facing complex patterns with more vertex labels.
DIAMNet also performs not so good over these patterns.
As for edge labels, results look good for MaxPool and DIAMNet but AttnPool is not satisfactory.
As shown in Figure 8 , different representation modules perform differently in different views.
CNN performs badly when the graph size is large (shown in Figure 8a and 8d) and patterns become complicated (show in Figure 8g and 8j), which further indicates that CNN can only extract the local information and suffers from issues when global information is need in larger graphs.
RNN, on the other hand, performs worse when the graph are large, especially when patterns are small (show in Figure 8e ), which is consistent with its nature, intuitively.
On the contrary, RGCN-SUM with DIAMNet is not affected by the edge sizes because it directly learns vertex representations rather than edge representations.

assistant: text: In this paper, we study a new graph learning problem: learning to count subgraph isomorphisms.