Geodesic
Operator $Af = f\circ\varphi-f$ in tasks of mathematical competitions
Matematičeskoe obrazovanie, Tome 97 (2021) no. 1, pp. 23-30.

Voir la notice de l'article provenant de la source Math-Net.Ru

The article proposes a certain operator point of view on some plots and constructions typical for Olympiad problems. It is shown that the usual classical Olympiad problems (for example, such as calculating some infinite products, finding certain sums of series or solutions to some functional equations) are particular manifestations of the general concept under consideration. 
@article{MO_2021_97_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},
     publisher = {mathdoc},
     volume = {97},
     number = {1},
     year = {2021},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MO_2021_97_1_a4/}
}
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E. I. Znak. Operator $Af = f\circ\varphi-f$ in tasks of mathematical competitions. Matematičeskoe obrazovanie, Tome 97 (2021) no. 1, pp. 23-30. http://geodesic.mathdoc.fr/item/MO_2021_97_1_a4/