Topic Detection and Segmentation in Automatic Text Summarization∗

A topic or a theme is what discourse, a discourse fragment or a sentence is about. It is the shortest summary of a discourse, the main proposition of a paragraph or what is commented on in a sentence. The term topic is usually defined as the aboutness of a unit of discourse [20]. Topic structure within a text can be tackled from two different points of view: from a Document Level (discourse topic) or a Sentence Level perspective (sentence topic). If a text is considered as a whole, it usually talks about a single topic, but in a more deeper analysis, several sub-topics can be identified, giving additional information about the main topic. On the other hand, taking into account sentence structure, we can find that every sentence has a topic (the part of the structure that is being presenting) and a comment (what is being asserted about the topic). Next, these two-way perspectives are going to be explained in more detail. Normally, a text deals with a single main topic, which is developed through the rest of the document, exposing several subtopics as well. It is important to detect these parts of text where a change of topic occurs. This leads to a hierarchical organization of a text into topics and subtopics, topic concatenation, and semantic return [1]. Text documents are usually subdivided in different paragraphs. A paragraph can be defined as a coherent text segment focused on a single topic. Every paragraph needs a topic sentence. The topic sentence is usually the first sentence of the paragraph [13]. It gives the reader an idea of what the paragraph is going to be about. The dicotomy of topic and focus is relevant not only for a possible placement of the sentence in a context, but also for its semantic interpretation [9]. The topic, also known as theme, can be understood as the part of the sentence structure that is being presented by the speaker as readily available in the hearer's memory; in other words, it is the " given " information, while the focus (comment or rheme) reflects