In mathematics, a sequence is an ordered list of objects (or events). Like a set, it contains members (also called elements, or terms), and the number of ordered element (possibly infinite) is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence. A sequence is a discrete function.
(C, R, Y) is a sequence of letters that differs from (Y, C, R), as the ordering matters Sequences can be finite, as in this example, or infinite, such as the sequence of all even positive integers (2, 4, 6,...).
In analysis, when talking about sequences, one will generally consider sequences of the form (X1, X2, X3, …..) or (X0, X1, X2,….) which is to say, infinite sequences of elements indexed by natural numbers.