“…Nonstationarity is a very important empirical feature in many economic and financial time series. Over the past decade, there has been great interests in nonparametric and semiparametric models with non stationary covariates, existing literature includes Cai, Li, and Park (2009), Chan and Wang (2015), Chen, Fang, and Li (2015), Chen, Gao, and Li (2012), Dong, Gao, and Tjøstheim (2016), Gu and Liang (2014), Gao and Phillips (2013), Juhl and Xiao (2005), Karlsen, Myklebust, and Tjøstheim (2007), Karlsen and Tjostheim (2001), Liang, Lin, and Hsiao (2015), Li et al (2017), Sun, Cai, and Li (2013), Sun and Li (2011), Wang (2014), Wang (2015), Wang and Phillips (2009a), Wang and Phillips (2009b), Wang and Phillips (2016), Xiao (2009), Zhou and Lin (2018). As we know, compared with nonparametric regression model, semiparametric regression models have the advantage of attenuating the problem of "curse of dimensionality. "…”