There are a lot of interesting-looking Soviet-era mathematics and science books and booklets (translated into English and free to read) at archive.org/details/@mirtitles

Via mathstodon.xyz/@jarban/1022097, where @jarban linked to one on linear recurrence relations.

A Mastodon instance for maths people. The kind of people who make $\pi z^2 \times a$ jokes.

Use $ and $ for inline LaTeX, and $ and $ for display mode.