planar numbers?

[James A. Sellers]

If I am not mistaken, I have seen this type of problem in an
undergraduate discrete math text.  I can't remember if it is Alan
Tucker's "Applied Combinatorics" or some other one.  My apologies for
not being more specific, but I am many miles from my books right now,
so I can't be more helpful.  I remember asking my most recent graph
theory students on an exam this past year to draw such a graph (based
on divisibility), and think I got the idea from a well-known text that
sits on my shelf.  

I am also not completely sure that it was a directed graph. 
Certainly, Neil's question is a good one, but I don't think you have
to make the graph directed in order to make sense of this idea. 

James 

>>> "N. J. A. Sloane" <njas at research.att.com> 07/25/00 21:41 PM >>>

Simon, isn't your graph really a directed graph?

join d1 with an arrow to d2 iff d1 divides d2 ?

Neil Sloane