RichardKennaway comments on Confound it! Correlation is (usually) not causation! But why not? - Less Wrong

44 Post author: gwern 09 July 2014 03:04AM

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Comment author: RichardKennaway 09 July 2014 10:27:44AM *  9 points [-]

Since a graph of n vertices has n choose 2 pairs, the total number of DAGs of n vertices has an upper bound of 3^(n choose 2). This is much smaller than n^n.

It is much larger. = , and is much larger than n.

3^(10 choose 2) is about 10^21.

Since the nodes of these graphs are all distinguishable, there is no need to factor out by graph isomorphism, so 3^(n choose 2) is the exact number.

Comment author: [deleted] 09 July 2014 02:59:21PM 6 points [-]

The precise asymptotic is , as shown on page 4 of this article. Here lambda and omega are constants between 1 and 2.

Comment author: mcallisterjp 09 July 2014 12:50:30PM 5 points [-]

That's the number of all directed graphs, some of which certainly have cycles.

Comment author: RichardKennaway 09 July 2014 01:08:45PM 6 points [-]

That's the number of all directed graphs, some of which certainly have cycles.

So it is. 3^(n choose 2) >> n^n stands though.

A lower bound for the number of DAGs can be found by observing that if we drop the directedness of the edges, there are 2^(n choose 2) undirected graphs on a set of n distinguishable vertices, and each of these corresponds to at least 1 DAG. Therefore there are at least that many DAGs, and 2^(n choose 2) is also much larger than n.

Comment author: IlyaShpitser 09 July 2014 03:48:47PM 4 points [-]

Yup you are right, re: what is larger.