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  2. 21 (number) - Wikipedia

    en.wikipedia.org/wiki/21_(number)

    21 is also the first non-trivial octagonal number. [5] It is the fifth Motzkin number, [6] and the seventeenth Padovan number (preceded by the terms 9, 12, and 16, where it is the sum of the first two of these). [7] In decimal, the number of two-digit prime numbers is twenty-one (a base in which 21 is the fourteenth Harshad number).

  3. List of Mersenne primes and perfect numbers - Wikipedia

    en.wikipedia.org/wiki/List_of_Mersenne_primes...

    Mersenne primes and perfect numbers are two deeply interlinked types of natural numbers in number theory. Mersenne primes, named after the friar Marin Mersenne, are prime numbers that can be expressed as 2 p − 1 for some positive integer p. For example, 3 is a Mersenne prime as it is a prime number and is expressible as 2 2 − 1.

  4. List of numeral systems - Wikipedia

    en.wikipedia.org/wiki/List_of_numeral_systems

    A form of unary notation called Church encoding is used to represent numbers within lambda calculus. Some email spam filters tag messages with a number of asterisks in an e-mail header such as X-Spam-Bar or X-SPAM-LEVEL. The larger the number, the more likely the email is considered spam. 10: Bijective base-10: To avoid zero: 26: Bijective base-26

  5. Number theory - Wikipedia

    en.wikipedia.org/wiki/Number_theory

    Number theory is a branch of pure mathematics devoted primarily to the study of the integers and ... propositions 21–34 are very probably influenced by ...

  6. Numbers (spreadsheet) - Wikipedia

    en.wikipedia.org/wiki/Numbers_(spreadsheet)

    Numbers is a spreadsheet application developed by Apple Inc. as part of the iWork productivity suite alongside Keynote and Pages. [2] Numbers is available for iOS and macOS High Sierra or newer. [3] Numbers 1.0 on Mac OS X was announced on August 7, 2007, making it the newest application in the iWork suite.

  7. Mathematics - Wikipedia

    en.wikipedia.org/wiki/Mathematics

    Number theory was once called arithmetic, ... [21] [22] The resulting Euclidean geometry is the study of shapes and their arrangements constructed from lines, ...