Hájek, Petr and Johanis, Michal. "Smooth approximations without critical points" Open Mathematics, vol. 1, no. 3, 2003, pp. 284-291. https://doi.org/10.2478/BF02475210
Hájek, P. & Johanis, M. (2003). Smooth approximations without critical points. Open Mathematics, 1(3), 284-291. https://doi.org/10.2478/BF02475210
Hájek, P. and Johanis, M. (2003) Smooth approximations without critical points. Open Mathematics, Vol. 1 (Issue 3), pp. 284-291. https://doi.org/10.2478/BF02475210
Hájek, Petr and Johanis, Michal. "Smooth approximations without critical points" Open Mathematics 1, no. 3 (2003): 284-291. https://doi.org/10.2478/BF02475210
Hájek P, Johanis M. Smooth approximations without critical points. Open Mathematics. 2003;1(3): 284-291. https://doi.org/10.2478/BF02475210