The definition of the Lyapunov exponent is
$\lambda = \lim_{n\rightarrow \infty}\left \{ \frac{1}{n} \sum_{i=0}^{n-1} ln\left | f'\left ( x_{i} \right ) \right |\right \}$.
A point $x_{i}$ is superstable if the Lyapunov exponent tends to negative infinity when n tends to infinity. By construction, it would appear that for this to be true, it must be true that $ln\left | f'\left ( x_{i} \right ) \right |<0$ for all $i \in \left [ 0,n-1 \right ]$ so that $\left | f'\left ( x_{i} \right ) \right | < 1$.
However, in my notes, the notes insists that $\left | f'\left ( x_{i} \right ) \right | \leq 0$ which to me looks rather silly.
Firstly, if $\left | f'\left ( x_{i} \right ) \right | =0$, the Lyapunov exponent will never be negative. Secondly, if $\left | f'\left ( x_{i} \right ) \right | < 0$, then, ln$ \left | f'\left ( x_{i} \right ) \right |$ is an imaginary number which is nonsensical.
Any clarification is much appreciated. Thanks in advance.