我查了下 rugarch包里面的函数ugarchfit以及ugarchspec，貌似只能拟合arma-garch模型，
ugarchspec是设定拟合模型的各个参数，官方解释如下：


ugarchspec(variance.model = list(model = "sGARCH", garchOrder = c(1, 1),submodel = NULL, external.regressors = NULL, variance.targeting = FALSE),mean.model = list(armaOrder = c(1, 1), include.mean = TRUE, archm = FALSE,archpow = 1, arfima = FALSE, external.regressors = NULL, archex = FALSE),distribution.model = "norm", start.pars = list(), fixed.pars = list(), ...)


里面的mean.model = list(armaOrder = c(1, 1)我理解应该就是设定AR、MA参数， 可是如何如何设定差分阶数呢？


或者R里面有别的更好用的包可以拟合ARIMA-GARCH模型？


谢谢