Inequality Constraints in the Univariate GARCH Model
指出Bollerslev对GARCH模型参数的非负约束可以大幅放宽，不应在估计中强制施加，并用实证例子说明放松约束的重要性。
To keep the conditional variances generated by the GARCH (p, q) model nonnegative, Bollerslev imposed nonnegativity constraints on the parameters of the process. We show that these constraints can be substantially weakened and so should not be imposed in estimation. We also provide empirical examples illustrating the importance of relaxing these constraints.