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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.
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 $\ …
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 …
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 $ …
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 …
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 …
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 …