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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 …
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 …
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 …
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 …
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 …
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 …
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 …