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  2. Divisibility rule - Wikipedia

    en.wikipedia.org/wiki/Divisibility_rule

    It is divisible by 2 and by 13. [6] 156: it is divisible by 2 and by 13. Subtracting 5 times the last digit from 2 times the rest of the number gives a multiple of 26. (Works because 52 is divisible by 26.) 1,248 : (124 × 2) − (8 × 5) = 208 = 26 × 8. 27: Sum the digits in blocks of three from right to left. (Works because 999 is divisible ...

  3. Proth's theorem - Wikipedia

    en.wikipedia.org/wiki/Proth's_theorem

    In number theory, Proth's theorem is a primality test for Proth numbers.. It states [1] [2] that if p is a Proth number, of the form k2 n + 1 with k odd and k < 2 n, and if there exists an integer a for which

  4. Euler brick - Wikipedia

    en.wikipedia.org/wiki/Euler_brick

    One edge must have length divisible by 11. One edge must have length divisible by 19. One edge or space diagonal must be divisible by 13. One edge, face diagonal or space diagonal must be divisible by 17. One edge, face diagonal or space diagonal must be divisible by 29. One edge, face diagonal or space diagonal must be divisible by 37. In ...

  5. 1001 (number) - Wikipedia

    en.wikipedia.org/wiki/1001_(number)

    Two properties of 1001 are the basis of a divisibility test for 7, 11 and 13. The method is along the same lines as the divisibility rule for 11 using the property 10 ≡ -1 (mod 11). The two properties of 1001 are 1001 = 7 × 11 × 13 in prime factors 10 3 ≡ -1 (mod 1001) The method simultaneously tests for divisibility by any of the factors ...

  6. Euler's totient function - Wikipedia

    en.wikipedia.org/wiki/Euler's_totient_function

    In words: the distinct prime factors of 20 are 2 and 5; half of the twenty integers from 1 to 20 are divisible by 2, leaving ten; a fifth of those are divisible by 5, leaving eight numbers coprime to 20; these are: 1, 3, 7, 9, 11, 13, 17, 19.

  7. When will Swift's birthday fall on Friday the 13th again? - AOL

    www.aol.com/friday-13th-week-another-one...

    Global popstar Taylor Swift was born on Dec. 13, 1989. This year, her birthday will be extra special - Friday the 13th. Why is the day considered bad? ... (which is also divisible by 13) in 2041.

  8. 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

  9. Does Trump’s penny plan make cents? How ditching the hated ...

    www.aol.com/finance/does-trump-penny-plan-cents...

    Since the U.S. loses more money producing nickels than it does pennies already—a nickel cost 13.78 cents to produce in 2024, according to the U.S. Mint—the U.S.’s financial burden would ...