D-Number

A natural number  such that

whenever  ( and  are relatively prime) and . (Here,  means that  divides .) There are an infinite number of such numbers, the first few being 9, 15, 21, 33, 39, 51, ... (OEIS A033553).

The -numbers are the same as the 3-Knödel numbers.

REFERENCES:

Makowski, A. "Generalization of Morrow's -Numbers." Simon Stevin 36, 71, 1962/1963.

Sloane, N. J. A. Sequence A033553 in "The On-Line Encyclopedia of Integer Sequences."