You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Review of Text Watermarking: Theory, Methods, and Applications 
Chapter Title:  1) HE et al. (2009) 
1. Abstract of Cited Paper (BIB001):  In order to promote the Chinese digital text's copyright protection technology, this paper proposes a Chinese Text Zero-Watermark scenario based on Sentence's Entropy. This scenario calculates the entropies of sentences based on wordfrequency and makes crucial sentence selection based on entropy. After that, watermark is constructed with the order of crucial sentences. Furthermore, to prove its effectiveness, the scenario is validated by simulation and it has shown fine robust and ability of anti-attack.

Literature Review:
The authors proposed a novel text zero watermarking algorithm which use words that correspond to one special partof-speech (POS) tag subsequence pattern. Part-of-speech is a category of words that have similar grammatical properties. The sequences were extracted using the chaotic function to develop the watermark without modifying the cover data, which resolves the problem of imperceptibility. This method offers good security since any attacker does not know the sequence of the selected POS tag while the chaotic function has unpredictable properties. Robustness is also assured and includes protection against reformatting and converting document type, synonym substitution, and sentence transformation attack with a very low success percentage BIB001 . The team discussed a method where the watermark key is calculated based on the sentence entropy. Entropy is the average expected value of the information contained in each message. The entropy of the sentence is calculated based on word frequency and crucial selections, where the VOLUME 6, 2018 watermark is constructed based on the order of the crucial sentences. Given the complexity of Chinese text semantics, a sentence with a high word frequency possesses bigger entropy. This method is tested for some known attacks which includes 'adding and deleting' attacks and synonym substitution attacks. Tests have shown that this method is robust and withstands this kind of attacks which shows very low success rate. This method offers imperceptibility and security where the watermark key is registered with the authority BIB002 .