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  2. Divisibility rule - Wikipedia

    en.wikipedia.org/wiki/Divisibility_rule

    12: It is divisible by 3 and by 4. [6] 324: it is divisible by 3 and by 4. Subtract the last digit from twice the rest. The result must be divisible by 12. 324: 32 × 2 − 4 = 60 = 5 × 12. 13: Form the alternating sum of blocks of three from right to left. The result must be divisible by 13. [7] 2,911,272: 272 − 911 + 2 = −637

  3. Vigesimal - Wikipedia

    en.wikipedia.org/wiki/Vigesimal

    D0 20 is equivalent to two hundred and sixty in decimal = (13 × 20 1) + (0 × 20 0) 100 20 is equivalent to four hundred in decimal = (1 × 20 2) + (0 × 20 1) + (0 × 20 0). In the rest of this article below, numbers are expressed in decimal notation, unless specified otherwise. For example, 10 means ten, 20 means twenty. Numbers in vigesimal ...

  4. Century common year - Wikipedia

    en.wikipedia.org/wiki/Century_common_year

    These years are the only common years that are divisible by 4. In the obsolete Julian Calendar, all years that were divisible by 4 were leap years, meaning no century years could be common years. However, this rule adds too many leap days, resulting in the calendar drifting with respect to the seasons, which is the same thing that would happen ...

  5. Table of divisors - Wikipedia

    en.wikipedia.org/wiki/Table_of_divisors

    d() is the number of positive divisors of n, including 1 and n itself; σ() is the sum of the positive divisors of n, including 1 and n itselfs() is the sum of the proper divisors of n, including 1 but not n itself; that is, s(n) = σ(n) − n

  6. Division lattice - Wikipedia

    en.wikipedia.org/wiki/Division_lattice

    The prime numbers are precisely the atoms of the division lattice, namely those natural numbers divisible only by themselves and 1. [2] For any square-free number n, its divisors form a Boolean algebra that is a sublattice of the division lattice. The elements of this sublattice are representable as the subsets of the set of prime factors of n. [3]

  7. 12 (number) - Wikipedia

    en.wikipedia.org/wiki/12_(number)

    12 (twelve) is the natural number following 11 and preceding 13.. Twelve is the 3rd superior highly composite number, [1] the 3rd colossally abundant number, [2] the 5th highly composite number, and is divisible by the numbers from 1 to 4, and 6, a large number of divisors comparatively.

  8. Singly and doubly even - Wikipedia

    en.wikipedia.org/wiki/Singly_and_doubly_even

    In mathematics an even integer, that is, a number that is divisible by 2, is called evenly even or doubly even if it is a multiple of 4, and oddly even or singly even if it is not. The former names are traditional ones, derived from ancient Greek mathematics ; the latter have become common in recent decades.

  9. Harshad number - Wikipedia

    en.wikipedia.org/wiki/Harshad_number

    The number 18 is a harshad number in base 10, because the sum of the digits 1 and 8 is 9, and 18 is divisible by 9.; The Hardy–Ramanujan number (1729) is a harshad number in base 10, since it is divisible by 19, the sum of its digits (1729 = 19 × 91).