Examining the correctness of the attribution of letter 53 of Nahj al-Balaghah to Imam Ali (PUH) Based on the most accurate theories of Vocabulary Richness

One of the most controversial issues in the present century is the existence of texts attributed to important people, for which there are no documents. Researchers have always tried to study these claims from different angles. The importance of this matter led to the emergence of a trend in linguistics, called "Authorship Attribution" (A.A.). Authorship Attribution is the task of identifying the authors of contested or anonymous texts, and aims to identify the author of unknown texts, based on writing style. There are various theories in this field, but so far, no comprehensive, preventable, and one hundred percent reliable method has been presented. In this research, Yule's theory was combined with four other theories about vocabulary richness, to study the validity of the attribution of the letter 53 of Nahj al-Balaghah to Imam Ali (PUH), through the descriptive-analytical and statistical method. The results of the research indicate that in addition to the high accuracy for the calculations, the output of theories does not depend on the length of the text. Moreover, the results indicate that the variable W has the most important role in determining vocabulary richness, and its value for letter 53 is not very different from its value for the other selected letters, so it is proven that the author of letter 53 and the other letters was one person, and as a result, the doubt about Nahj al-Balaghah is false.