— Roman Dobrovensky, a mathematics textbook: logic, sets, combinatorics, algebra. A copy kept here: heller.ru does not answer, and the snapshot 
does not always open. — Roman Dobrovensky, a mathematics textbook: logic, sets, combinatorics, algebra. A copy kept here: heller.ru does not answer, and the snapshot does not always open. links to an Internet Archive snapshot — used when the original is gone, or reachable only from some countries.