References KPSS test

bhargava, a. (1986). on theory of testing unit roots in observed time series . review of economic studies. 53 (3): 369–384. doi:10.2307/2297634. jstor 2297634. 
kwiatkowski, d.; phillips, p. c. b.; schmidt, p.; shin, y. (1992). testing null hypothesis of stationarity against alternative of unit root . journal of econometrics. 54 (1–3): 159–178. doi:10.1016/0304-4076(92)90104-y.  (a free pdf of 1992 paper hosted here on deu.edu.tr)