6174 (number)

6174 is known as Kaprekar's constant[1][2][3] after the Indian mathematician D. R. Kaprekar. This number is notable for the following rule:

  1. Take any four-digit number, using at least two different digits (leading zeros are allowed).
  2. Arrange the digits in descending and then in ascending order to get two four-digit numbers, adding leading zeros if necessary.
  3. Subtract the smaller number from the bigger number.
  4. Go back to step 2 and repeat.
← 6173 6174 6175 →
← 0 [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]] [[{{#expr:{{{1}}}*{{{factor}}}*1000}} (number)|{{#ifeq:{{{1}}}|10|→|{{#expr:{{{1}}}*{{{factor}}}}}k}}]]
Cardinalsix thousand one hundred seventy-four
Ordinal6174th
(six thousand one hundred seventy-fourth)
Factorization2 × 32 × 73
Greek numeral,ϚΡΟΔ´
Roman numeralVMCLXXIV, or VICLXXIV
Binary11000000111102
Ternary221102003
Octal140368
Duodecimal36A612
Hexadecimal181E16

The above process, known as Kaprekar's routine, will always reach its fixed point, 6174, in at most 7 iterations.[4] Once 6174 is reached, the process will continue yielding 7641 – 1467 = 6174. For example, choose 1495:

9541 – 1459 = 8082
8820 – 0288 = 8532
8532 – 2358 = 6174
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.

Other "Kaprekar's constants"
Main article: Kaprekar's routine § Definition and properties

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".

Other properties
  • 6174 is a harshad number, since it is divisible by the sum of its digits:
  • 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 degrees of 18:
    183 + 182 + 181 = 5832 + 324 + 18 = 6174.
  • 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

References
  1. Nishiyama, Yutaka. "Mysterious number 6174". Plus Magazine.
  2. Kaprekar DR (1955). "An Interesting Property of the Number 6174". Scripta Mathematica. 15: 244–245.
  3. Kaprekar DR (1980). "On Kaprekar Numbers". Journal of Recreational Mathematics. 13 (2): 81–82.
  4. Weisstein, Eric W. "Kaprekar Routine". MathWorld.
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