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  2. Kaprekar's routine - Wikipedia

    en.wikipedia.org/wiki/Kaprekar's_routine

    For six-digit numbers, there are two solutions that satisfy equations (1) and (2). [9] Furthermore, it is clear that even-digits with greater than or equal to 8, [ 10 ] and with 9 digits, [ 11 ] or odd-digits with greater than or equal to 15 digits [ 12 ] have multiple solutions.

  3. List of integer sequences - Wikipedia

    en.wikipedia.org/wiki/List_of_integer_sequences

    Download QR code; Print/export ... 2, 145, 40585, ... A natural number that equals the sum of the factorials of its decimal digits. ... A number that has the digit ...

  4. Perfect digit-to-digit invariant - Wikipedia

    en.wikipedia.org/wiki/Perfect_digit-to-digit...

    Let be a natural number which can be written in base as the k-digit number ... where each digit is between and inclusive, and = =.We define the function : as () = =. (As 0 0 is usually undefined, there are typically two conventions used, one where it is taken to be equal to one, and another where it is taken to be equal to zero.

  5. Digital root - Wikipedia

    en.wikipedia.org/wiki/Digital_root

    The digital root (also repeated digital sum) of a natural number in a given radix is the (single digit) value obtained by an iterative process of summing digits, on each iteration using the result from the previous iteration to compute a digit sum. The process continues until a single-digit number is reached.

  6. Combinatorial number system - Wikipedia

    en.wikipedia.org/wiki/Combinatorial_number_system

    (this associates distinct numbers to all finite sets of natural numbers); then comparison of k-combinations can be done by comparing the associated binary numbers. In the example C and C′ correspond to numbers 1001011001 2 = 601 10 and 1010001011 2 = 651 10, which again shows that C comes before C′.

  7. Narcissistic number - Wikipedia

    en.wikipedia.org/wiki/Narcissistic_number

    In number theory, a narcissistic number [1] [2] (also known as a pluperfect digital invariant (PPDI), [3] an Armstrong number [4] (after Michael F. Armstrong) [5] or a plus perfect number) [6] in a given number base is a number that is the sum of its own digits each raised to the power of the number of digits.

  8. Gödel numbering - Wikipedia

    en.wikipedia.org/wiki/Gödel_numbering

    These sequences of natural numbers can again be represented by single natural numbers, facilitating their manipulation in formal theories of arithmetic. Since the publishing of Gödel's paper in 1931, the term "Gödel numbering" or "Gödel code" has been used to refer to more general assignments of natural numbers to mathematical objects.

  9. Middle-square method - Wikipedia

    en.wikipedia.org/wiki/Middle-square_method

    Consider the following: If a 3-digit number is squared, it can yield a 6-digit number (e.g. 540 2 = 291600). If there were to be middle 3 digits, that would leave 6 − 3 = 3 digits to be distributed to the left and right of the middle. It is impossible to evenly distribute these digits equally on both sides of the middle number, and therefore ...