The position of an element in a sequence is its "rank" or "index"; it is the natural number for which the element is the image. The first element has index 0 or 1, depending on the context or a specific convention. In mathematical analysis, a sequence is often denoted by letters in the form of formula_1, formula_2 and formula_3, where the subscript "n" refers to the "n"th element of the sequence; for example, the "n"th element of the Fibonacci sequence "formula_4" is generally denoted as "formula_5".