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adam_strandberg comments on Confound it! Correlation is (usually) not causation! But why not? - LessWrong
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Comments (34)
Naively, I would expect it to be closer to 600^600 (the number of possible directed graphs with 600 nodes).
And in fact, it is some complicated thing that seems to scale much more like n^n than like 2^n: http://en.wikipedia.org/wiki/Directed_acyclic_graph#Combinatorial_enumeration
There's an asymptotic approximation in the OEIS: a(n) ~ n!2^(n(n-1)/2)/(M*p^n), with M and p constants. So log(a(n)) = O(n^2), as opposed to log(2^n) = O(n), log(n!) = O(n log(n)), log(n^n) = O(n log(n)).
It appears I've accidentally nerdsniped everyone! I was just trying to give an idea that it was really really big. (I had done some googling for the exact answer but they all seemed rather complicated, and rather than try and get an exact answer wrong, just give a lower bound.)
If we allow cycles, then there are three possibilities for an edge between a pair of vertices in a directed graph: no edge, or an arrow in either direction. 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.
edit: the last sentence is wrong.
Gwern, thanks for writing more, I will have more to say later.
It is much larger.
= ^{n-1})^n)
, and ^{n-1})
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.
The precise asymptotic is%202%5E{\binom{n}{2}}%20\omega%5E{-n})
, as shown on page 4 of this article. Here lambda and omega are constants between 1 and 2.
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.
Yup you are right, re: what is larger.