Here is the problem:

Suppose a sequence is defined recursively be setting $\displaystyle a_1 = 2$ and $\displaystyle a_2 = 9$ and, for $\displaystyle n\geq2$ , requiring that $\displaystyle a_n = 2a_{n-1} + 3a_{n-2}$.

Give a recursive algorithm for computing $\displaystyle a_n$, where $\displaystyle n$ is a positive integer.

Using the initial conditions, I could calculate $\displaystyle a_3 , a_4$ , etc., but that is not what the problem is asking for. I thought recursive definitions were when you set $\displaystyle n = 0$, but that makes the equation work with negative numbers. So, here I am, confused beyond belief with algorithms