Consider an n-digit number k. Square it and add the right n digits to the left n or n - 1 digits. If the resultant sum is k, then k is called a Kaprekar number. For example, 9 is a Kaprekar number since 9^2 = 81 8 + 1 = 9, and 297 is a Kaprekar number since 297^2 = 88209 88 + 209 = 297. The first few are 1, 9, 45, 55, 99, 297, 703, ... (OEIS A006886).

digitaddition | digital root | happy number | Kaprekar routine | Kaprekar's constant | narcissistic number | recurring digital invariant

D. R. Kaprekar