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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.

7 votes

Dyck paths on rectangles

I heard a talk at Indiana University last March by Timothy Chow. Here's his abstract, which seems to give a negative answer to your question about rectangles whose sides have non-integer ratio: It i …
Will Orrick's user avatar
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0 votes

analog of principle of inclusion-exclusion

I'll take a stab at answering my own question. The missing “something” in the edited version of my question appears to be the mutual information of one or more events, denoted $I(A,B,C,...)$. More p …
Will Orrick's user avatar
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5 votes

Algorithmic Combinatorics resources?

The following web site might be useful to you: The Stony Brook Algorithm Repository, http://www.cs.sunysb.edu/~algorith/
2 votes

Algorithmic Combinatorics resources?

Another book is Classification Algorithms for Codes and Designs by Kaski and Östergård. The authors are responsible for the recent complete enumeration of Steiner triple systems of order 19. (All 11 …
2 votes

Algorithmic Combinatorics resources?

Here's an article available online: "Isomorph-free exhaustive generation" by Brendan D. McKay. See cs.anu.edu.au/~bdm/papers/orderly.pdf The author also wrote the graph isomorphism package 'nauty …
8 votes
Accepted

Binary matrices with constant row and column sums

To answer your question about interesting combinatorial objects: Your Sylvester-Hadamard matrix example generalizes in at least two ways. The incidence matrix of any balanced incomplete block design …
Will Orrick's user avatar
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16 votes
6 answers
3k views

analog of principle of inclusion-exclusion

When I teach elementary probability to my finite math students, a common error is to mix up the concepts of disjointness and independence. At some point I thought that it might be helpful to some stu …
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4 votes

Number of unique determinants for an NxN (0,1)-matrix

From Hadamard's bound the largest possible determinant of an $n\times n$ (0,1) matrix is $h_n=2^{-n}(n+1)^{(n+1)/2}$. The data at http://www.indiana.edu/~maxdet/spectrum.html suggest several conject …
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5 votes

[Matrices over Z] - An algorithm for calculating the diagonal with elementary operations

At Scott's request, here's my comment in answer form: The stated conditions imply imply that your two matrices are equivalent. Up to permutation of rows and columns, your diagonal matrix is the Smit …
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9 votes

Maximum determinant of $\{0,1\}$-valued $n\times n$-matrices

As pointed out in Robin Chapman's answer and Frederio Poloni's comment thereto, there is a one-to-one map between normalized $n$-by-$n$ $(-1,1)$ matrices and $(n-1)$-by-$(n-1)$ $(0,1)$ matrices under …
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