Conrad, Christian - In: Journal of Econometrics 157 (2010) 2, pp. 441-457
In this article we derive conditions which ensure the non-negativity of the conditional variance in the Hyperbolic GARCH(p,d,q) (HYGARCH) model of Davidson (2004). The conditions are necessary and sufficient for p=1 and sufficient for p=2 and emerge as natural extensions of the inequality...