user: text: Write a concise one-sentence TLDR summary of the scientific paper content below.

Title: Semi-supervised Autoencoding Projective Dependency Parsing

Paper content:
We describe two end-to-end autoencoding models for semi-supervised graph-based dependency parsing.
The first model is a Local Autoencoding Parser (LAP) encoding the input using continuous latent variables in a sequential manner; The second model is a Global Autoencoding Parser (GAP) encoding the input into dependency trees as latent variables, with exact inference.
Both models consist of two parts: an encoder enhanced by deep neural networks (DNN) that can utilize the contextual information to encode the input into latent variables, and a decoder which is a generative model able to reconstruct the input.
Both LAP and GAP admit a unified structure with different loss functions for labeled and unlabeled data with shared parameters.
We conducted experiments on WSJ and UD dependency parsing data sets, showing that our models can exploit the unlabeled data to boost the performance given a limited amount of labeled data.
Dependency parsing captures bi-lexical relationships by constructing directional arcs between words, defining a head-modifier syntactic structure for sentences, as shown in Figure 1 .
Dependency trees are fundamental for many downstream tasks such as semantic parsing (Reddy et al., 2016; , machine translation (Bastings et al., 2017; Ding & Palmer, 2007) , information extraction (Culotta & Sorensen, 2004; Liu et al., 2015) and question answering (Cui et al., 2005) .
As a result, efficient parsers (Kiperwasser & Goldberg, 2016; Ma et al., 2018) have been developed using various neural architectures.
While supervised approaches have been very successful, they require large amounts of labeled data, particularly when neural architectures are used.
Syntactic annotation is notoriously difficult and requires specialized linguistic expertise, posing a serious challenge for low-resource languages.
Semisupervised parsing aims to alleviate this problem by combining a small amount of labeled data and a large amount of unlabeled data, to improve parsing performance over labeled data alone.
Traditional semi-supervised parsers use unlabeled data to generate additional features, assisting the learning process (Koo et al., 2008) , together with different variants of self-training (Søgaard & Rishøj, 2010) .
However, these approaches are usually pipe-lined and error-propagation may occur.
In this paper, we propose two end-to-end semi-supervised parsers based on probabilistic autoencoder models illustrated in Figure 3 , Locally Autoencoding Parser (LAP) and Globally Autoencoding Parser (GAP).
In LAP, continuous latent variables are used to support tree inference by providing a better representation, while in GAP, the latent information forms a probability distribution over dependency trees corresponding to the input sentence.
A similar idea has been proposed by Corro & Titov (2018) , but our GAP model differs fundamentally from their parser, as GAP does not sample from the posterior of the latent tree structure to approximate the Evidence Lower Bound (ELBO).
Instead it relies on a tractable algorithm to directly compute the posterior to calculate the ELBO.
We summarize our contributions as follows:
1. We proposed two autoencoding parsers for semi-supervised dependency parsing, with complementary strengths, trading off speed vs. accuracy;
2. We propose a tractable inference algorithm to compute the expectation and marginalization of the latent dependency tree posterior analytically for GAP, avoiding sampling from the posterior to approximate the expectation (Corro & Titov, 2018)
; 3. We show improved performance of both LAP and GAP with unlabeled data on WSJ and UD data sets empirically, and improved results of GAP comparing to a recently proposed semi-supervised parser (Corro & Titov, 2018) .
In this paper, we present two semi-supervised parsers, which are locally autoencoding parser (LAP) and globally autoencoding parser (GAP).
Both of them are end-to-end learning systems enhanced with neural architecture, capable of utilizing the latent information within the unlabeled data together with labeled data to improve the parsing performance, without using external resources.
More importantly, our GAP model outperforms the previous published (Corro & Titov, 2018) semisupervised parsing system on the WSJ data set.
We attribute this success to two reasons: First, our GAP model consists both a discriminative component and a generative component.
These two components are constraining and supplementing each other such that final parsing choices are made in a checked-and-balanced manner to avoid over-fitting.
Second, instead of sampling from posterior of the latent variable (the dependency tree) (Corro & Titov, 2018) , our model analytically computes the expectation and marginalization of the latent variable, such that the global optima can be found for the decoder, which leads to an improved performance.
A APPENDIX

assistant: text: We describe two end-to-end autoencoding parsers for semi-supervised graph-based dependency parsing.