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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.

5 votes

Is the "diagonal" of a regular language always context-free?

It's unnecessary to assume that L is unambiguous: a regular language always is, because there exists a DFA that accepts it. Following Richard's notation, it is easy to construct a DPDA for K, so it i …
didest's user avatar
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4 votes
3 answers
662 views

Minimum cover of partitions of a set

Given $n,k\in\mathbb{N}$ where $k\leq n$, I want to compute the minimum subset of the set of partitions of $N$={$1,\ldots,n$}, satisfying these properties: Each block of every partition has at most …
didest's user avatar
  • 1,015
31 votes
Accepted

Exhibit an explicit bijection between irreducible polynomials over finite fields and Lyndon ...

In Reutenauer's "Free Lie Algebras", section 7.6.2: A direct bijection between primitive necklaces of length $n$ over $F$ and the set of irreducible polynomials of degree $n$ in $F[x]$ may be describe …
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  • 1,015
1 vote

How to compute the rook polynomial of a Ferrers board?

Although the closed formula is what I wanted, a dynamic programming approach behaves better algorithmically: Define $M_{i,j}$ as the number of ways to place $j$ non-attacking rooks on the Ferrers boa …
didest's user avatar
  • 1,015
4 votes
2 answers
2k views

How to compute the rook polynomial of a Ferrers board?

Given a Ferrers board of shape $(b_1,\ldots,b_m)$, we define $r_k$ as number of ways to place $k$ non-attacking rooks (as in Chess). In section 2.4 of Stanley's Enumerative Combinatorics (vol. 1) it's …
didest's user avatar
  • 1,015
14 votes
0 answers
4k views

Minimum tiling of a rectangle by squares

Given the $n\times m$ rectangle, I want to compute the minimum number of integer-sided squares needed to tile it (possibly of different sizes). Is there an efficient way to calculate this?
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