A 
-number is a natural
number 
such that
 |
for every integer 
with 
and 
(
and 
are relatively prime). (Here, 
means that 
divides 
.) There are infinitely many such numbers, the first few being
9, 15, 21, 33, 39, 51, ... (OEIS A033553).
The 
-numbers are the same as the 3-Knödel
numbers.
-Numbers." Simon Stevin 36, 71, 1962/1963.Sloane,
N. J. A. Sequence