“…The proposed models with properties i)-v) are different from the models in existing literature ( [1], [6], [8], [9], [10], [14], [15], [16], [19], [20], [21]). For instance, the models based on the Ornstein-Uhlenbeck processes and their superpositions ( [19], [21]) do not satisfy the property i), while the model proposed in [6] uses the Student distribution with time-dependent parameters. The model in [8], [15] uses the Student distribution with only two parameters.…”