# Math Insight

### Recurrence relation definition

A recurrence relation is an equation that defines a sequence based on a rule that gives the next term as a function of the previous term(s).

The simplest form of a recurrence relation is the case where the next term depends only on the immediately previous term. If we denote the $n$th term in the sequence by $x_n$, such a recurrence relation is of the form \begin{gather*} x_{n+1}=f(x_n) \end{gather*} for some function $f$. One such example is $x_{n+1}=2-x_n/2.$

A recurrence relation can also be higher order, where the term $x_{n+1}$ could depend not only on the previous term $x_n$ but also on earlier terms such as $x_{n-1}$, $x_{n-2}$, etc. A second order recurrence relation depends just on $x_n$ and $x_{n-1}$ and is of the form \begin{gather*} x_{n+1}=f(x_n,x_{n-1}) \end{gather*} for some function $f$ with two inputs. For example, the recurrence relation $x_{n+1}=x_n + x_{n-1}$ can generate the Fibonacci numbers.

To generate sequence basd on a recurrence relation, one must start with some initial values. For a first order recursion $x_{n+1}=f(x_n)$, one just needs to start with an initial value $x_0$ and can generate all remaining terms using the recurrence relation. For a second order recursion $x_{n+1}=f(x_n,x_{n-1})$, one needs to begin with two values $x_0$ and $x_1$. Higher order recurrence relations require correspondingly more initial values.

A recurrence relation can be viewed as determining a discrete dynamical system.