The moduli space of n points on the line is cut out by simple quadrics when n is not six

Benjamin Howard; John Millson; Andrew Snowden; Ravi Vakil

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The moduli space of n points on the line is cut out by simple quadrics when n is not six

Benjamin Howard·John Millson·Andrew Snowden·Ravi Vakil

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Abstract

A central question in invariant theory is that of determining the relations among invariants. Geometric invariant theory quotients come with a natural ample line bundle, and hence often a natural projective embedding. This question translates to determining the equations of the moduli space under this embedding. This note deals with one of the most classical quotients, the space of ordered points on the projective line. We show that under any linearization, this quotient is cut out (scheme-theoretically) by a particularly simple set of quadric relations, with the single exception of the Segre cubic threefold (the space of six points with equal weight). Unlike many facts in geometric invariant theory, these results (at least for the stable locus) are field-independent, and indeed work over the integers.

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Benjamin Howard, John Millson, Andrew Snowden, Ravi Vakil. 2006-07-16. The moduli space of n points on the line is cut out by simple quadrics when n is not six. https://arxiv.org/abs/math/0607372

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