“…Subsequently, Lee et al (2015), on the other hand, developed a panel version of the Fourier DF‐type test, which allows smooth transitions in the deterministic term, which they modeled with the Fourier function, following the approach of Enders and Lee (2012b). Lee et al (2015), the Fourier Panel KPSS test, developed by Nazlıoğlu and Karul (2017), is a test that detects possible soft‐transition breaks and considers cross‐section dependence with the bootstrap approach, allowing heterogeneity between cross‐sections (Erdoğan, Akalın, & Oypan, 2020). This test provides an advantage by combining soft structural breaks with intrinsic hard structural breaks (Tiwari et al, 2021).…”