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2. 24 (puzzle) - Wikipedia

   en.wikipedia.org/wiki/24_(puzzle)

   The original version of 24 is played with an ordinary deck of playing cards with all the face cards removed. The aces are taken to have the value 1 and the basic game proceeds by having 4 cards dealt and the first player that can achieve the number 24 exactly using only allowed operations (addition, subtraction, multiplication, division, and parentheses) wins the hand.

3. Surreal number - Wikipedia

   en.wikipedia.org/wiki/Surreal_number

   Addition, negation, and comparison are all defined the same way for both surreal numbers and games. Every surreal number is a game, but not all games are surreal numbers, e.g. the game { 0 | 0} is not a surreal number. The class of games is more general than the surreals, and has a simpler definition, but lacks some of the nicer properties of ...

4. Multiply perfect number - Wikipedia

   en.wikipedia.org/wiki/Multiply_perfect_number

   In mathematics, a multiply perfect number (also called multiperfect number or pluperfect number) is a generalization of a perfect number. For a given natural number k , a number n is called k -perfect (or k -fold perfect) if the sum of all positive divisors of n (the divisor function , σ ( n )) is equal to kn ; a number is thus perfect if and ...

5. Perfect number - Wikipedia

   en.wikipedia.org/wiki/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.

6. Arithmetic - Wikipedia

   en.wikipedia.org/wiki/Arithmetic

   Arithmetic is an elementary branch of mathematics that studies numerical operations like addition, subtraction, multiplication, and division. In a wider sense, it also includes exponentiation, extraction of roots, and taking logarithms. Arithmetic systems can be distinguished based on the type of numbers they operate on.

7. Friendly number - Wikipedia

   en.wikipedia.org/wiki/Friendly_number

   The smallest friendly number is 6, forming for example, the friendly pair 6 and 28 with abundancy σ(6) / 6 = (1+2+3+6) / 6 = 2, the same as σ(28) / 28 = (1+2+4+7+14+28) / 28 = 2. The shared value 2 is an integer in this case but not in many other cases. Numbers with abundancy 2 are also known as perfect numbers. There are several unsolved ...