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  2. Four fours - Wikipedia

    en.wikipedia.org/wiki/Four_fours

    For example, when d=4, the hash table for two occurrences of d would contain the key-value pair 8 and 4+4, and the one for three occurrences, the key-value pair 2 and (4+4)/4 (strings shown in bold). The task is then reduced to recursively computing these hash tables for increasing n , starting from n=1 and continuing up to e.g. n=4.

  3. Perfect number - Wikipedia

    en.wikipedia.org/wiki/Perfect_number

    Perfect number. In number theory, a perfect number is a positive integer that is equal to the sum of its positive proper divisors, that is, divisors excluding the number itself. For instance, 6 has proper divisors 1, 2 and 3, and 1 + 2 + 3 = 6, so 6 is a perfect number. The next perfect number is 28, since 1 + 2 + 4 + 7 + 14 = 28.

  4. Multiplication table - Wikipedia

    en.wikipedia.org/wiki/Multiplication_table

    Cycles of the unit digit of multiples of integers ending in 1, 3, 7 and 9 (upper row), and 2, 4, 6 and 8 (lower row) on a telephone keypad. Figure 1 is used for multiples of 1, 3, 7, and 9. Figure 2 is used for the multiples of 2, 4, 6, and 8. These patterns can be used to memorize the multiples of any number from 0 to 10, except 5.

  5. Product (mathematics) - Wikipedia

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

    Product (mathematics) In mathematics, a product is the result of multiplication, or an expression that identifies objects (numbers or variables) to be multiplied, called factors. For example, 21 is the product of 3 and 7 (the result of multiplication), and is the product of and (indicating that the two factors should be multiplied together).

  6. Fraction - Wikipedia

    en.wikipedia.org/wiki/Fraction

    A simple fraction (also known as a common fraction or vulgar fraction, where vulgar is Latin for "common") is a rational number written as a / b or ⁠ ⁠, where a and b are both integers. [9] As with other fractions, the denominator (b) cannot be zero. Examples include ⁠ 1 2 ⁠, − ⁠ 8 5 ⁠, ⁠ −8 5 ⁠, and ⁠ 8 −5 ⁠.

  7. Lagrange's four-square theorem - Wikipedia

    en.wikipedia.org/wiki/Lagrange's_four-square_theorem

    Lagrange's four-square theorem, also known as Bachet's conjecture, states that every natural number can be represented as a sum of four non-negative integer squares. [ 1] That is, the squares form an additive basis of order four. where the four numbers are integers. For illustration, 3, 31, and 310 in several ways, can be represented as the sum ...

  8. Divisibility rule - Wikipedia

    en.wikipedia.org/wiki/Divisibility_rule

    The sum of the ones digit, double the tens digit, four times the hundreds digit, and eight times the thousands digit is divisible by 16. 157,648: 7 × 8 + 6 × 4 + 4 × 2 + 8 = 96 17: Subtract 5 times the last digit from the rest. (Works because 51 is divisible by 17.) 221: 22 − 1 × 5 = 17. Add 12 times the last digit to the rest.

  9. Numeral prefix - Wikipedia

    en.wikipedia.org/wiki/Numeral_prefix

    Numeral or number prefixes are prefixes derived from numerals or occasionally other numbers. In English and many other languages, they are used to coin numerous series of words. For example: simplex, duplex (communication in only 1 direction at a time, in 2 directions simultaneously) unicycle, bicycle, tricycle (vehicle with 1 wheel, 2 wheels ...