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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.

14 votes

What exact number of domino tilings cannot be realizable?

Consider a $4\times 2k+1$ rectangle. Label its points by $(x,y)$, with $x$ from $0$ to $3$ and $y$ from $0$ to $2k.$ Remove from the rectangle the points $(1,y)$ and $(2,y)$ for $y$ odd. I claim that …
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13 votes

Alice and Bob playing on a circle

Here is an argument that Alice wins for odd $n$. Label the points $1$ to $n.$ Lemma: If at any point Alice has marked $k$ points in a row and no other points, Bob has marked the two points immediatel …
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9 votes
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Simple question about polynomials

This equation has no solutions when $d$ is odd. (EDIT: See below for the general case.) Actually, for $d$ odd, there are no triples $(F_0,F_1,F_2)$ with $F_1\cdot F_2-F_0^2$ a multiple of $x$, let al …
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