[2] While still at school, he worked at Shanyavsky University on an experimental project on X-ray radiation and was supervised by Petr Lazarev.

[1] By 1922, Urysohn had given topological definitions to curve, surface, and dimension, and his work attracted the attention of many prominent European mathematicians.

[1][4] In the summers of 1923 and 1924, Urysohn and his friend and fellow mathematician, Pavel Aleksandrov, traveled through France, Holland, and Germany, where they met David Hilbert, Felix Hausdorff, and L. E. J.

[1] The three European mathematicians were impressed by Urysohn’s work and expressed their hopes that he would return to Europe in subsequent years.

Not being a mathematician, she included in the book memorial articles about his mathematical works by Pavel Alexandrov, Vadim Efremovich, Andrei Kolmogorov, Lazar Lyusternik, and Mark Krasnosel'skii.