If a sequence is finite, it is easy to say what it is: one can simply write down all the things in the sequence. This does not work for an infinite sequence. So another way to write down a sequence is to write a rule for finding the thing in any place one wants. The rule should tell us how to get the thing in the "n"-th place, where "n" can be any natural number. This means that a sequence is really a special kind of function with natural numbers as its domain. We sometimes write a sequence as formula_1, where formula_2 stands for the "n"-th term of the sequence.