Other notations can be useful for sequences whose pattern cannot be easily guessed, or for sequences that do not have a pattern such as the digits of pi. One such notation is to write down a general formula for computing the "n"th term as a function of "n", enclose it in parentheses, and include a subscript indicating the set of values that "n" can take. For example, in this notation the sequence of even numbers could be written as formula_6. The sequence of squares could be written as formula_7. The variable "n" is called an index, and the set of values that it can take is called the index set.