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Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.

9 votes
Accepted

How many edge-disjoint paths go from upper left to lower right in a $4 \times N$ rectangular...

To amplify on Christian's answer, the problem on a $K \times N$ grid for fixed $K$ and varying $N$ admits a finite-state transition model, so in particular is given by a linear recurrence. The key is …
Dylan Thurston's user avatar
12 votes
0 answers
261 views

Computing the number of ways to delete vertices sequentially without disconnecting a graph

Given a finite connected graph on $n$ vertices, we are trying to count the number of ways to label the vertices $1$ to $n$ so that deleting them sequentially in that order never disconnects the graph. …
Dylan Thurston's user avatar
3 votes

characterization of regular languages among (say) those computable in linear time

(Answering Lukasz's question from the comments) If you consider the non-commutative power series for a language, the regular languages are characterized by having rational power series constructable …
Dylan Thurston's user avatar
1 vote

If the series Σ pᵃ⁽ʷ⁾·xᴵʷᴵ is rational, is Σ a(w)·xᴵʷᴵ also rational (summation over words ...

If there are only finitely many values for the $a_i$, then I believe the answer to both questions is affirmative for somewhat silly reasons. If you have any rational power series $f(x)$ with only fi …
Dylan Thurston's user avatar