FAQ

On Thu, 2009-02-19 at 22:57 +1100, Timothy S. Nelson wrote:
On Thu, 19 Feb 2009, Carl Mäsak wrote:
A tree is a graph without cycles.
That's insufficient. In fact, there are a number of ways that the
general concept of an acyclic graph must be constrained before you get
something you can call a 'tree'.
The concept of a "root" is very common in computer representations, but in
no way necessary for a general tree. In fact, in phylogenetics, it's
business as usual to handle unrooted trees. This makes the $root attribute
meaningless in at least some cases.
Interesting. I'm happy to assume that $root is allowed to be
Undefined, I think. But let me ask a question; were you to represent an
unrooted tree in a computer, how would you do it so that, if you had to look
around the tree, you could do it? You'd need some node that was an
entry-point into the tree. That's the purpose I'm trying to get at here.
A tree with nodes but without a root is not a tree -- it's a collection
of trees, more commonly called a grove or forest.


-'f

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postedFeb 18, '09 at 5:44a
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