\sum_{k = 0}^{\infty} \left( \frac{\left( -1 \right)^{k} \cdot \left( a^{2 \cdot k + 1} - \left( -a \right)^{2 \cdot k + 1} \right)}{k! \cdot \left( 2 \cdot k + 1 \right)} \right)