Six thousand one hundred seventy-four — Kaprekar's constant

Take any 4-digit number (not all digits the same), form the largest and smallest numbers from its digits, subtract the smaller from the larger. Repeat — and within 7 steps you will get 6174. Always. Discovered by mathematician D.R. Kaprekar in 1949.

Kaprekar's algorithm is astonishing: from the chaos of any number, order inevitably emerges — 6174. This process is called 'Kaprekar's routine'. The number 6174 is the only such 'black hole' for four-digit numbers.

D.R. Kaprekar (1905–1986) was an Indian recreational mathematician who discovered a number of remarkable numerical phenomena. Besides the constant 6174, he described Kaprekar numbers (e.g., 9: 9² = 81, 8+1 = 9). He was recognized only posthumously.