olemus
täisarvude aritmeetika, milles tehted on moodultehted:
tavalise liitmis- või korrutustehte tulem asendatakse jäägiga,
mis tekib ta jagamisel mingi arvuga n, st mooduliga

Wiktionary:
(number theory) any system of arithmetic for integers which, for some given positive integer n, is equivalent to the set of integers being mapped onto the finite set {0, ... n} according to congruence modulo n, and in which addition and multiplication are defined consistently with the results of ordinary arithmetic being so mapped

ülevaateid
https://betterexplained.com/articles/fun-with-modular-arithmetic/

http://mathworld.wolfram.com/ModularArithmetic.html

https://en.wikipedia.org/wiki/Modular_arithmetic

https://www.khanacademy.org/computing/computer-science/cryptography/modarithmetic/a/what-is-modular-arithmetic

https://davidaltizio.web.illinois.edu/ModularArithmetic.pdf

https://www2.math.upenn.edu/~mlazar/math170/notes06-2.pdf

rakendusi
avaliku võtmega krüptograafia