\begin{definition}[Definition:Fibonacci String]
Consider the alphabet $\left\{ {\text {a}, \text {b} }\right\}$.
For all $n \in \Z_{>0}$, let $S_n$ be the (finite) string formed as:
:$S_n = \begin{cases} \text {a} & : n = 1 \\
\text {b} & : n = 2 \\
S_{n - 1} S_{n - 2} & : n > 2 \end{cases}$
where $S_{n - 1} S_{n - 2}$ denotes that $S_{n - 1}$ and $S_{n - 2}$ are concatenated.
The terms of the sequence $\left\langle{S_n}\right\rangle$ are '''Fibonacci strings'''.
\end{definition}