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Re: [igraph] cohesive.blocks()

From: MATSUDA, Noriyuki
Subject: Re: [igraph] cohesive.blocks()
Date: Wed, 30 Apr 2008 18:54:28 +0900


   I know cohesive.blocks() extracts two blocks that are cliques.
      block4:   {1-7}
      block 5:  {7,8,11,14)

But, I have an impression, from the explanations about Figures 2
and 3 in Moody and White's paper, another one is embedded in
the figures:
        {17,18,19,20,21,22,23}  (not a clique)
   This is derivable from the output of cohesive.blocks().  Yet I'd
be happy if the group were directly obtainable, if it is cohesive (
to a lesser extent than a clique).

    I know that  a set {8, 9, 10, 11, 12, 14, 15} is not cohesive.

On 2008/04/28, at 23:30, Gabor Csardi wrote:


to be honest, i don't really understand the question.
In your graph (==block 1, cohesion 1) the algorithm finds
two sub-blocks (block 2, cohesion 2, and block 3, cohesion 2).
In block 2 there is a (more) cohesive sub-block, this
will be block 4, it has cohesion 5. Within block 3, there is
a 3-cohesive block, this will be block 5.

In the last case, the remainder of block 3, i.e. the vertices
not in block 5 (vertices 8, 9, 10, 11, 12, 14, 15) do not
constitute a cohesive block, that's why they are not included in
the result.

If in a B cohesive block there is a sub-block C with higher
cohesion value, that does not mean that B-C (the remainder)
has a higher cohesion value (than the original B block) as well.

Does this answer your question?

On Mon, Apr 28, 2008 at 05:54:20PM +0900, MATSUDA, Noriyuki wrote:

   I have some difficulty in relating the performance of
cohesive.blocks() to the original paper by Moody and White (2003),
particularly the concepts of components and blocks.  Here is my trial
on Figures 2 and 3 in the paper:
  It separates terminal (bottom) blocks 4 and 5, leaving  {17-23}
unextracted.   Do we need to identify {17-23} by taking the
difference between blocks 2 and 4?  If so, it would require a lot of
care even with a graph of medium size.  Or have I made errors in
specifying edges?
  Thanks in advance for helping me out.


Csardi Gabor <address@hidden>    UNIL DGM

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MATSUDA, Noriyuki  <Kanji> 松田紀之

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