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Title: Jointly Learning Sentence Embeddings and Syntax with Unsupervised Tree-LSTMs

Paper content:
We introduce a neural network that represents sentences by composing their words according to induced binary parse trees.
We use Tree-LSTM as our composition function, applied along a tree structure found by a fully differentiable natural language chart parser.
Our model simultaneously optimises both the composition function and the parser, thus eliminating the need for externally-provided parse trees which are normally required for Tree-LSTM.
It can therefore be seen as a tree-based RNN that is unsupervised with respect to the parse trees.
As it is fully differentiable, our model is easily trained with an off-the-shelf gradient descent method and backpropagation.
We demonstrate that it achieves better performance compared to various supervised Tree-LSTM architectures on a textual entailment task and a reverse dictionary task.
Finally, we show how performance can be improved with an attention mechanism which fully exploits the parse chart, by attending over all possible subspans of the sentence.
Recurrent neural networks, in particular the Long Short-Term Memory (LSTM) architecture BID10 and some of its variants BID8 BID1 , have been widely applied to problems in natural language processing.
Examples include language modelling BID35 BID13 , textual entailment BID2 BID30 , and machine translation BID1 BID36 amongst others.The topology of an LSTM network is linear: words are read sequentially, normally in left-to-right order.
However, language is known to have an underlying hierarchical, tree-like structure BID4 .
How to capture this structure in a neural network, and whether doing so leads to improved performance on common linguistic tasks, is an open question.
The Tree-LSTM network BID37 BID41 provides a possible answer, by generalising the LSTM to tree-structured topologies.
It was shown to be more effective than a standard LSTM in semantic relatedness and sentiment analysis tasks.Despite their superior performance on these tasks, Tree-LSTM networks have the drawback of requiring an extra labelling of the input sentences in the form of parse trees.
These can be either provided by an automatic parser BID37 , or taken from a gold-standard resource such as the Penn Treebank BID18 .
BID39 proposed to remove this requirement by including a shift-reduce parser in the model, to be optimised alongside the composition function based on a downstream task.
This makes the full model non-differentiable so it needs to be trained with reinforcement learning, which can be slow due to high variance.Our proposed approach is to include a fully differentiable chart parser in the model, inspired by the CYK constituency parser BID5 BID40 BID15 .
Due to the parser being differentiable, the entire model can be trained end-to-end for a downstream task by using stochastic gradient descent.
Our model is also unsupervised with respect to the parse trees, similar to BID39 .
We show that the proposed method outperforms baseline Tree-LSTM architectures based on fully left-branching, right-branching, and supervised parse trees on a textual entailment task and a reverse dictionary task.
We also introduce an attention mechanism in the spirit of BID1 for our model, which attends over all possible subspans of the source sentence via the parse chart.
The results in TAB1 show a strong performance of the Unsupervised Tree-LSTM against our tested baselines, as well as other similar methods in the literature with a comparable number of parameters.For the textual entailment task, our model outperforms all baselines including the supervised Tree-LSTM, as well as some of the other sentence embedding models in the literature with a higher number of parameters.
The use of attention, extended for the Unsupervised Tree-LSTM to be over all possible subspans, further improves performance.
In the reverse dictionary task, the poor performance of the supervised Tree-LSTM can be explained by the unusual tokenisation used in the dataset of BID9 : punctuation is simply stripped, turning e.g. "(archaic) a section of a poem" into "archaic a section of a poem", or stripping away the semicolons in long lists of synonyms.
On the one hand, this might seem unfair on the supervised Tree-LSTM, which received suboptimal trees as input.
On the other hand, it demonstrates the robustness of our method to noisy data.
Our model also performed well in comparison to the LSTM and the other Tree-LSTM baselines.
Despite the slower training time due to the additional complexity, FIG2 shows how our model needed fewer training examples to reach convergence in this task.Following BID39 , we also manually inspect the learned trees to see how closely they match conventional syntax trees, as would typically be assigned by trained linguists.
We analyse the same four sentences they chose.
The trees produced by our model are shown in Figure 3 .
One notable feature is the fact that verbs are joined with their subject noun phrases first, which differs from the standard verb phrase structure.
However, formalisms such as combinatory categorial grammar BID34 , through type-raising and composition operators, do allow such constituents.
The spans of prepositional phrases in
(b),
(c) and
(d) are correctly identified at the highest level; but only in
(d) does the structure of the subtree match convention.
As could be expected, other features such as the attachment of the full stops or of some determiners do not appear to match human intuition.
We presented a fully differentiable model to jointly learn sentence embeddings and syntax, based on the Tree-LSTM composition function.
We demonstrated its benefits over standard Tree-LSTM on a textual entailment task and a reverse dictionary task.
Introducing an attention mechanism over the parse chart was shown to further improve performance for the textual entailment task.
The model is conceptually simple, and easy to train via backpropagation and stochastic gradient descent with popular deep learning toolkits based on dynamic computation graphs such as DyNet BID26 and PyTorch.
The unsupervised Tree-LSTM we presented is relatively simple, but could be plausibly improved by combining it with aspects of other models.
It should be noted in particular that (4), the function assigning an energy to alternative ways of forming constituents, is extremely basic and does not rely on any global information on the sentence.
Using a more complex function, perhaps relying on a mechanism such as the tracking LSTM in BID3 , might lead to improvements in performance.
Techniques such as batch normalization BID11 or layer normalization BID0 might also lead to further improvements.In future work, it may be possible to obtain trees closer to human intuition by training models to perform well on multiple tasks instead of a single one, an important feature for intelligent agents to demonstrate BID21 .
Elastic weight consolidation BID19 has been shown to help with multitask learning, and could be readily applied to our model.

assistant: text: Represent sentences by composing them with Tree-LSTMs according to automatically induced parse trees.