# Sequence

## Finite or infinite ordered list of elements / From Wikipedia, the free encyclopedia

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In mathematics, a **sequence** is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called *elements*, or *terms*). The number of elements (possibly infinite) is called the *length* of the sequence. Unlike a set, the same elements can appear multiple times at different positions in a sequence, and unlike a set, the order does matter. Formally, a sequence can be defined as a function from natural numbers (the positions of elements in the sequence) to the elements at each position. The notion of a sequence can be generalized to an indexed family, defined as a function from an *arbitrary* index set.

For example, (M, A, R, Y) is a sequence of letters with the letter 'M' first and 'Y' last. This sequence differs from (A, R, M, Y). Also, the sequence (1, 1, 2, 3, 5, 8), which contains the number 1 at two different positions, is a valid sequence. Sequences can be *finite*, as in these examples, or *infinite*, such as the sequence of all even positive integers (2, 4, 6, ...).

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 $a_{n}$, $b_{n}$ and $c_{n}$, where the subscript *n* refers to the *n*th element of the sequence; for example, the *n*th element of the Fibonacci sequence *$F$* is generally denoted as *$F_{n}$*.

In computing and computer science, finite sequences are usually called *strings*, *words* or *lists* - with the specific technical term chosen depending on the type of object the sequence enumerates and the different ways to represent the sequence in computer memory. Infinite sequences are called *streams*.

The empty sequence ( ) is included in most notions of sequence. It may be excluded depending on the context.