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  2. Divisibility rule - Wikipedia

    en.wikipedia.org/wiki/Divisibility_rule

    Subtracting 2 times the last digit from the rest gives a multiple of 3. (Works because 21 is divisible by 3) 405: 40 - 5 x 2 = 40 - 10 = 30 = 3 x 10 4: The last two digits form a number that is divisible by 4. [2] [3] 40,832: 32 is divisible by 4. If the tens digit is even, the ones digit must be 0, 4, or 8.

  3. Division (mathematics) - Wikipedia

    en.wikipedia.org/wiki/Division_(mathematics)

    This is denoted as 20 / 5 = 4, or ⁠ 20 / 5 ⁠ = 4. [2] In the example, 20 is the dividend, 5 is the divisor, and 4 is the quotient. Unlike the other basic operations, when dividing natural numbers there is sometimes a remainder that will not go evenly into the dividend; for example, 10 / 3 leaves a remainder of 1, as 10 is not a multiple of 3.

  4. Casting out nines - Wikipedia

    en.wikipedia.org/wiki/Casting_out_nines

    To "cast out nines" from a single number, its decimal digits can be simply added together to obtain its so-called digit sum. The digit sum of 2946, for example is 2 + 9 + 4 + 6 = 21. Since 21 = 2946 − 325 × 9, the effect of taking the digit sum of 2946 is to "cast out" 325 lots of 9 from it. If the digit 9 is ignored when summing the digits ...

  5. Senary - Wikipedia

    en.wikipedia.org/wiki/Senary

    If a number is divisible by 2, then the final digit of that number, when expressed in senary, is 0, 2, or 4. If a number is divisible by 3, then the final digit of that number in senary is 0 or 3. A number is divisible by 4 if its penultimate digit is odd and its final digit is 2, or its penultimate digit is even and its final digit is 0 or 4.

  6. Prime number - Wikipedia

    en.wikipedia.org/wiki/Prime_number

    A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. However, 4 is composite because it is a ...

  7. Collatz conjecture - Wikipedia

    en.wikipedia.org/wiki/Collatz_conjecture

    For instance, the first counterexample must be odd because f(2n) = n, smaller than 2n; and it must be 3 mod 4 because f 2 (4n + 1) = 3n + 1, smaller than 4n + 1. For each starting value a which is not a counterexample to the Collatz conjecture, there is a k for which such an inequality holds, so checking the Collatz conjecture for one starting ...

  8. Table of prime factors - Wikipedia

    en.wikipedia.org/wiki/Table_of_prime_factors

    The first: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23 (sequence A005408 in the OEIS). All integers are either even or odd. All integers are either even or odd. A square has even multiplicity for all prime factors (it is of the form a 2 for some a ).

  9. Division by zero - Wikipedia

    en.wikipedia.org/wiki/Division_by_zero

    Division is the inverse of multiplication, meaning that multiplying and then dividing by the same non-zero quantity, or vice versa, leaves an original quantity unchanged; for example () / = (/) =. [12]

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