https://anton-petrunin.github.io/puzzles/ https://arxiv.org/abs/0906.0290 для долгих зимних вечеров — книга PIGTIKAL («puzzles in geometry that I know and love») А.Петрунина «This collection is about ideas, and it is not about theory. An idea might feel more comfortable in a suitable theory, but it has its own life and history, and it can speak for itself. I am collecting these problems for fun, but they might be used to improve the problem-solving skills in geometry. Every problem has a short elegant solution — this gives a hint which was not available when the problem was discovered.»