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6174 - Wikipedia

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Natural number

Natural number

6174 (six thousand, one hundred [and] seventy-four) is the natural number following 6173 and preceding 6175.

The natural integer 6174 is known as Kaprekar's constant,^[1][2][3] after the Indian mathematician D. R. Kaprekar. This number is notable for the following curious behavior:

1. Select any four-digit number that has at least two different digits (leading zeros are allowed),
2. Create two new four-digit numbers by arranging the original digits in a.) ascending and b.) descending order (adding leading zeros if necessary).
3. Subtract the smaller number from the bigger number.
4. If the result is not 6174, return to step 2 and repeat.

This process, known as Kaprekar's routine, is guaranteed to reach a fixed point at the value 6174 in no more than 7 iterations,^[4] at which point it will continue yielding that value (7641 - 1467 = 6174).

The only four-digit numbers for which Kaprekar's routine does not reach 6174 are repdigits such as 1111, which give the result 0000 after a single iteration. All other four-digit numbers eventually reach 6174 if leading zeros are used to keep the number of digits at 4. For numbers with three identical digits and a fourth digit that is one higher or lower (such as 2111), it is essential to treat 3-digit numbers with a leading zero; for example: 2111 – 1112 = 0999; 9990 – 999 = 8991; 9981 – 1899 = 8082; 8820 – 288 = 8532; 8532 – 2358 = 6174.^[5]

There can be analogous fixed points for digit lengths other than four; for instance, if we use 3-digit numbers, then most sequences (i.e., other than repdigits such as 111) will terminate in the value 495 in at most 6 iterations. Sometimes these numbers (495, 6174, and their counterparts in other digit lengths or in bases other than 10) are called "Kaprekar constants".

In numerical analysis, Kaprekar's constant can be used to analyze the convergence of a variety of numerical methods. Numerical methods are used in engineering, various forms of calculus, coding, and many other mathematical and scientific fields.

The properties of Kaprekar's routine allow for the study of recursive functions, ones which repeat previous values and generate sequences based on these values. Kaprekar's routine is a recursive arithmetic sequence, so it helps study the properties of recursive functions. ^[6]

• 6174 is a 7-smooth number, i.e. none of its prime factors are greater than 7.
• 6174 can be written as the sum of the first three powers of 18:
  • 18³ + 18² + 18¹ = 5832 + 324 + 18 = 6174, and coincidentally, 6 + 1 + 7 + 4 = 18.
• The sum of squares of the prime factors of 6174 is a square:
  • 2² + 3² + 3² + 7² + 7² + 7² = 4 + 9 + 9 + 49 + 49 + 49 = 169 = 13²

• Bowley, Roger (5 December 2011). "6174 is Kaprekar's Constant". Numberphile. University of Nottingham: Brady Haran.
• Sample (Perl) code to walk any four-digit number to Kaprekar's Constant
• Sample (Python) code to walk any four-digit number to Kaprekar's Constant
• Sample (C) code to walk the first 10000 numbers and their steps to Kaprekar's Constant

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