@article{MO_2021_1_a4,
     author = {E. I. Znak},
     title = {Operator $Af = f\circ\varphi-f$ in tasks of mathematical competitions},
     journal = {Matemati\v{c}eskoe obrazovanie},
     pages = {23--30},
     year = {2021},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MO_2021_1_a4/}
}