A nialpdrome is a number whose hexadecimal digits are in nonincreasing order. The first few are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 32, 33, 34, 48, 49, 50, ... (Sloane's A023771), corresponding to 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 20, 21, 22, 30, 31, 32, ....

A number that is not a nialpdrome is a metadrome.

The following table summarized related classes of numbers.

Sloane, N. J. A. Sequence A023771 in "The On-Line Encyclopedia of Integer Sequences."

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