A sequence is either a finite or infinite list of numbers which are called terms. One example of a sequence is $$1,4,7,10,13,\dots$$ The dots imply that the sequence continues infinitely. The terms are often represented as $a_1,$ $a_2,$ $a_3$, and so on, where the numbers at the bottom right of the letters are called indices and specify the position of the terms in the sequence. In the example, $$a_1=1,a_2=4,a_3=7,\text{etc.}$$ Several terms can have the same value, even though this is not the case here. Two specific types of sequences are arithmetic sequences and geometric sequences.