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From Pascal’s Theorem to d-Constructible Curves

Will Traves
The American Mathematical Monthly
Vol. 120, No. 10 (December 2013), pp. 901-915
DOI: 10.4169/amer.math.monthly.120.10.901
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Page Count: 15
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From Pascal’s Theorem to d-Constructible Curves
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Abstract We prove a generalization of both Pascal’s Theorem and its converse, the Braikenridge–Maclaurin Theorem: If two sets of k lines meet in k2 distinct points, and if dk of those points lie on an irreducible curve C of degree d, then the remaining k(k − d) points lie on a unique curve S of degree k − d. If S is a curve of degree k − d produced in this manner using a curve C of degree d, we say that S is d-constructible. For fixed degree d, we show that almost every curve of high degree is not d-constructible. In contrast, almost all curves of degree 3 or less are d-constructible. The proof of this last result uses the group structure on an elliptic curve and is inspired by a construction due to Möbius. The exposition is embellished with several exercises designed to amuse the reader.

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