Paper:
Neural networks have been shown to be an effective tool for learning algorithms over graph-structured data. However, graph representation techniques-that convert graphs to real-valued vectors for use with neural networks-are still in their infancy. Recent works have proposed several approaches (e.g., graph convolutional networks), but these methods have difficulty scaling and generalizing to graphs with different sizes and shapes. We present GRAPH2SEQ, a new technique that represents vertices of graphs as infinite time-series. By not limiting the representation to a fixed dimension, GRAPH2SEQ scales naturally to graphs of arbitrary sizes and shapes. GRAPH2SEQ is also reversible, allowing full recovery of the graph structure from the sequences. By analyzing a formal computational model for graph representation, we show that an unbounded sequence is necessary for scalability. Our experimental results with GRAPH2SEQ show strong generalization and new state-of-the-art performance on a variety of graph combinatorial optimization problems.

Review:
This paper proposes a new representation learning model for graph optimization, Graph2Seq. The novelty of Graph2Seq lies in utilizing intermediate vector representation of vertices in the final representation. Theoretically, the authors show that an infinite sequence of such intermediate representations is much more powerful than existing models, which do not maintain intermediate representations. Experimentally, Graph2Seq results in greedy heuristics that generalize very well from small training graphs (e.g. 15 nodes) to large testing graphs (e.g. 3200 nodes).
Overall, the current version of the paper raises a number of crucial questions that I would like the authors to address before I make my decision.
First, some strengths of the paper:
- Theory: although I have not reviewed the proofs in details, the theorems are very interesting. If correct, the theorems provide a strong basis for Graph2Seq. In contrast, this aspect is missing from other work on ML for optimization.
- Experiments: the experiments are generally thorough and well-presented. The performance of Graph2Seq is remarkable, especially in terms of generalization to significantly larger graphs.
- Writing: the paper is very well-written and complex ideas are neatly articulated. I also liked the Appendix trying to interpret the trained model. Good job!
That being said, I have some serious concerns. Please clarify if I misunderstood anything and update the paper otherwise.
- Graph2Seq at test time: in section Testing, you explain how multiple solutions are output by G2S-RNN at intermediate "states" of the model, and the best w.r.t. the objective value is returned. If I understand all this correctly, you take the output of the T-th LSTM unit, run it through the Q-network, then select the next node (e.g. in a vertex cover solution). Then, the complexity should be O((E+V)*T_max*V), since the Graph2Seq operations are linear in the size of the graph O(E+V), a single G2S-RNN(i) takes O(V) times if you want to construct a cover of size O(V), and you repeat that process exactly T_max times, for each i between 1 and T_max. What's wrong in my understanding of G2S-RNN here? Or is your complexity incorrect?
- Local-Gather definition: in your definition of the Local-Gather model, do you assume that computations are performed for a single iteration, i.e. a single local step followed by a gather step? If so, then how is Graph2Seq infinity-local-gather? What does that even mean? I understand how some of the other GCNN-based models like Khalil et al.'s is 4-local-gather (assuming 4 embedding iterations of structure2vec), but how is Graph2Seq infinity-local-gather?
- Comparison to Structure2Vec: for fair comparison, why not apply Algorithm 2 to that method? Just run more embedding iterations up to T_max, and use the best among the solutions constructed between 1 and T_max.
Minor:
- Section 4: Vinyals et al. (2015) does not do any RL.