Updown Numbers and the Initial Monomials of the Slope Variety
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Let $I_n$ be the ideal of all algebraic relations on the slopes of the ${n\choose2}$ 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 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.
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1970 ◽
Vol 28
◽
pp. 174-175
1974 ◽
Vol 32
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pp. 330-331
1978 ◽
Vol 36
(1)
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pp. 222-223
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1978 ◽
Vol 36
(1)
◽
pp. 100-101
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1994 ◽
Vol 52
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pp. 992-993
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