A katadrome is a number whose hexadecimal digits are in strict descending order. The first few are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 32, 33, 48, 49, ... (Sloane's A023797), corresponding to 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 20, 21, 30, 31, ....

A number that is not a katadrome is a plaindrome.

The following table summarized related classes of numbers.

Sloane, N. J. A. Sequence A023797 in "The On-Line Encyclopedia of Integer Sequences."

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Weisstein, Eric W. "Katadrome." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Katadrome.html