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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.
1
vote
Complement to part of a permutation matrix
It's basically a coset? You have an injective function to extend to a permutation. If it were the identity on the first m elements you'd have the n - m other elements to permute. So if you multiply on …
1
vote
A system of equations for integers
Here something is being said about the generating functions r(t) and c(t) for the $R_{k}$ and $C_{k}$; we'd better agree to begin with that $R_{0}$ and $C_{0}$ for that purpose are both equal to 1. Yo …
3
votes
Determining a recurrence relation
I'd certainly expect to acquire some useful information by modular arithmetic, in a given situation. For example modulo a prime p, if there is a linear recurrence, then the sequence of residues must a …
2
votes
Integers in a triangle, and differences
Not an answer, but a question about constraints mod 2. There you are just taking differences, and the number 0s and 1s must be the same for n that is 0 or 3 mod 4 (one different in the other two cases …
1
vote
Order of a combination when mapping them to whole numbers
Well, yes, in the sense that dividing 26 by 12 choose 2 immediately tells you that 0 is the first digit. Dividing 26 by 11 choose 1 indicates that the second digit is 3. Subtracting 12 + 11 from 26 in …
3
votes
A family of subsets with a "gluing" property
It's just an exercise in the transitivity of relations? You have the subsets where a given function F takes a given value: in other words any partition, or any equivalence relation. I wouldn't call th …
1
vote
Accepted
Number of config. of a binary string invariant under cyclic permutation.
http://en.wikipedia.org/wiki/Necklace_(combinatorics) will get you started.