Before the table of contents. It is a brief statement of the content of the paper, without comments and annotations. It needs to briefly explain the purpose of the research work, and you need to get the catalogue before you can write an abstract. Papers with a table of contents are very long. For the convenience of readers, the main paragraphs and chapters will be written into a table of contents and marked in front of the main body of the paper, so that others can know what the paper is about when searching for the paper.