The notation a_n-1 represents a term in a sequence or series, specifically the term that comes before the n -th term. In this context, n is an index that typically denotes the position of a term within the sequence. For example, if n = 5 , then a_n-1 would refer to a_4 , the fourth term. This notation is commonly used in mathematics, particularly in areas such as algebra, calculus, and number theory. It helps in defining recursive sequences, where each term is derived from previous terms. Understanding a_n-1 is essential for analyzing patterns and behaviors in sequences.