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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.
24
votes
Order of product of group elements
The following theorem (which does not take the order $N$ of the group $G$ into account) shows that all possible combinations of $a$, $b$ and the order of $xy$ are possible. See Theorem 1.64 from Milne …
11
votes
rational function identity
I'm not sure whether my answer is conceptual in your sense, but here is a relatively short proof. First of all, your definition of $f$ suggests the notation
$$s_p := \sum_{i=p}^n x_i.$$
Now consider t …
10
votes
Accepted
Which finite projective planes can have a symmetric incidence matrix?
The key word here is "polarity". A polarity of a projective plane with point set $P$ and line set $L$ is a map $\pi$ from $P \cup L$ to itself mapping points to lines and lines to points, such that $\ …
4
votes
Accepted
The Symmetry of Steiner System S(5,8,24)
If you're only interested in finite permutation groups, then Koen S has given you the answer you needed. If you allow infinite objects, then there are much more symmetric objects than S(5,8,24).
In f …
2
votes
Accepted
Actions of $Z_n$ and actions of $Z_{n-1}$
I might be missing something, but it seems to me that there is not much going on in your construction. In fact, your original action of $Z_n$ on $X$ does nothing more than putting a cyclic ordering on …
1
vote
Accepted
Non-trivial alternating sums of binomial coefficients
(Reposting my comment as an answer, as requested by the OP.)
If you have a solution with $a_i \in \{−1,1\}$, then you also have a solution with $a_i \in \{0,1\}$ simply by replacing each $a_i$ with $ …