There are a lot of interesting-looking Soviet-era mathematics and science books and booklets (translated into English and free to read) at https://archive.org/details/@mirtitles
Via https://mathstodon.xyz/@jarban/102209797748141088, 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.
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