Incidences between points and curves with almost two degrees of freedom
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We study incidences between points and algebraic curves in three dimensions, taken from a family C of curves that have almost two degrees of freedom, meaning that every pair of curves intersect in O(1) points, for any pair of points p, q, there are only O(1) curves of C that pass through both points, and a pair p, q of points admit a curve of C that passes through both of them iff F(p,q)=0 for some polynomial F. We study two specific instances, one involving unit circles in R^3 that pass through some fixed point (so called anchored unit circles), and the other involving tangencies between directed points (points and directions) and circles in the plane; a directed point is tangent to a circle if the point lies on the circle and the direction is the tangent direction. A lifting transformation of Ellenberg et al. maps these tangencies to incidences between points and curves in three dimensions. In both instances the curves in R^3 have almost two degrees of freedom. We show that the number of incidences between m points and n anchored unit circles in R^3, as well as the number of tangencies between m directed points and n arbitrary circles in the plane, is O(m^3/5n^3/5+m+n). We derive a similar incidence bound, with a few additional terms, for more general families of curves in R^3 with almost two degrees of freedom. The proofs follow standard techniques, based on polynomial partitioning, but face a novel issue involving surfaces that are infinitely ruled by the respective family of curves, as well as surfaces in a dual 3D space that are infinitely ruled by the respective family of suitably defined dual curves. The general bound that we obtain is O(m^3/5n^3/5+m+n) plus additional terms that depend on how many curves or dual curves can lie on an infinitely-ruled surface.
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