Kaprekar

A Kaprekar number (for a given base) is a number that, when squared, can be split into two parts that sum to the original number. For example, 45² = 2025 and 20 + 25 = 45. Named after D. R. Kaprekar.

Key features

• 1 and 9 are Kaprekar numbers (9² = 81, 8 + 1 = 9).
• Definition depends on how the square is split (e.g. 0-padding).

Examples:

1, 9, 45, 55, 99, 297

Further reading

• Kaprekar on Wikipedia
• Kaprekar on OEIS