Definition:  for a sinusoidal quantity $a(t)=\widehat{A}\text{cos}(\omega \text{\hspace{0.05em}}t+{\vartheta}_{0})$, complex value $\underset{\_}{\widehat{A}}=\widehat{A}\text{exp}\mathrm{j}{\vartheta}_{0}$, where j is the imaginary unit, $\omega $ is angular frequency, and ${\vartheta}_{0}$ is initial phase
