Notes: All unit root tests include an intercept and trend; If null hypotheses are θ₁ = θ₂ = 0, there is unit root; Model 2 is estimated, as in Kruse (2011) test, using error terms from Model 1; k* indicates the optimal frequency, while k indicates the optimal number of lags; Table critical values of unit root tests are taken from Guris (2019). The table critical values of the FKruse unit root test are 23.56, 18.54, 15.74 for k* = 1 and 18.78, 14.2, 12.32 for k* = 2 and Becker et al. (2006) F table value is 6.87, 4.97 and 3.92 for 1%, 5% and 10%, respectively; In case, the unit root null hypothesis is rejected, Becker et al. (2006) used the F statistic to test the null hypothesis of the absence of the non-linear trend suggested in the study, and the rejection of the null hypothesis states that the series is stationary around the broken deterministic function;

*** and

** imply rejection of the unit root at the 1% and 5% levels, respectively