Simulation, estimation and inference for univariate and multivariate
TV(s)-GARCH(p,q,r)-X models, where s indicates the number and shape of the
transition functions, p is the ARCH order, q is the GARCH order, r is the
asymmetry order, and 'X' indicates that covariates can be included; see
Campos-Martins and Sucarrat (2024) <doi:10.18637/jss.v108.i09>. The TV
long-term component, as in the multiplicative TV-GARCH model of Amado and
Terasvirta (2013) <doi:10.1016/j.jeconom.2013.03.006>, introduces
non-stationarity whereas the GARCH-X short-term component describes
conditional heteroscedasticity. Maximisation by parts leads to consistent
and asymptotically normal estimates. In the multivariate case, conditional
variances are estimated equation by equation and dynamic conditional
correlations are allowed.