A sequence is either a finite or infinite list of numbers which are called terms. One example of a sequence is $1,\, 4, \, 7, \, 10, \, 13, \ldots$ 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, $a_1=1, \quad a_2=4, \quad a_3=7, \ \text{etc.}$ 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.