The function returns the vector \(\Psi(\theta,\tau,pm)\) defined as \(E(e^{ix\tau}\), where \(\tau\) is a vector of real numbers, \(i\) is the imaginary number, \(x\) is a stable random variable with parameters \(\theta\) = \((\alpha,\beta,\gamma,\delta)\) and pm
is the type of parametrization. The vector of parameters are the characteristic exponent, the skewness, the scale and the location parameters, respectively. The restrictions on the parameters are: \(\alpha \in (0,2]\), \(\beta\in [-1,1]\) and \(\gamma>0\). For mode details see Nolan(2009).