Sometimes instead of using different variables for different values, an index is used with the same variable. To distinguish between values, the index is written as a subscript to the term.

Indices can be used to specify multiple solutions of an equation or unknown terms of a sequence. a_1, a_2, a_3, ..., a_n Since there is a finite number of letters, writing these unknown terms in this way, rather than a, b, c,..., allows more flexibility in expressing the sequence.