f_x(a) = \left\{ 
\begin{array}{l l}
 b_a & \quad \mbox{ako je $a \in \mathbb{N}_0$}\\
 0 & \quad \mbox{ako je $a \leq -2$}\\
 -1 & \quad \mbox{ako je $a=-1$}\\
\end{array} \right.