Mathematics – Combinatorics
Scientific paper
2009-05-28
Electronic J. Combin. 16, no. 1 (2009), R82
Mathematics
Combinatorics
7 pages; 1 figure
Scientific paper
Let $I_n$ be the ideal of all algebraic relations on the slopes of the $\binom{n}{2}$ lines formed by placing $n$ points in a plane and connecting each pair of points with a line. Under each of two natural term orders, the initial ideal of $I_n$ is generated by monomials corresponding to permutations satisfying a certain pattern-avoidance condition. We show bijectively that these permutations are enumerated by the updown (or Euler) numbers, thereby obtaining a formula for the number of generators of the initial ideal of $I_n$ in each degree.
Martin Jeremy L.
Wagner Jennifer D.
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