Although sequences are a type of function, they are usually distinguished notationally from functions in that the input is written as a subscript rather than in parentheses, i.e. an rather than f(n). There are terminological differences as well: the value of a sequence at the input 1 is called the "first element" of the sequence, the value at 2 is called the "second element", etc. Also, while a function abstracted from its input is usually denoted by a single letter, e.g. f, a sequence abstracted from its input is usually written by a notation such as {\displaystyle (a_{n})_{n\in A}} , or just as {\displaystyle (a_{n})} . Here A is the domain, or index set, of the sequence. sequential numbering in word