sequence

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). Unlike a set, the same elements can appear multiple times at different positions in a sequence, and unlike a set, the order does matter. 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 even positive integers (2, 4, 6, ...), meaning that each element is twice the value of its position.
The length of a finite sequence is defined as the number of elements in the sequence. 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




{\displaystyle a_{n}}

,




b

n




{\displaystyle b_{n}}

and




c

n




{\displaystyle c_{n}}

, where the subscript n refers to the nth element of the sequence; for example, the nth element of the Fibonacci sequence



F


{\displaystyle F}

is generally denoted as




F

n




{\displaystyle 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.

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