A recursive sequence.
For example, the Fibonacci sequence is a sequence that is generated by adding two starting values together to create a third value; then the fourth is the sum of the second and third, the fifth is the sum of the third and fourth, and so on.
Recursively, it's defined as
[tex]\begin{cases}F_1=1\\F_2=1\\F_{n+2}=F_{n+1}+F_n&\text{for }n\ge3\end{cases}[/tex]
