New paper up on the arXiv, with Jozsef Solymosi and Josh Zahl. Suppose you have n plane curves of bounded degree. There ought to be about n^2 intersections between them. But there are intersections and there are intersections! Generically, an intersection between two curves is a node. But maybe the curves are mutually tangent at a point — that’s a more intense kind of singularity called a *tacnode*. You might think, well, OK, a tacnode is just some singularity of bounded multiplicity, so maybe there could still be a constant multiple of n^2 mutual tangencies.

No! In fact, we show there are O(n^{3/2}). (Megyesi and Szabo had previously given an upper bound of the form n^{2-delta} in the case where the curves are all conics.)

Is n^{3/2} best possible? Good question. The best known lower bound is given by a configuration of n circles with about n^{4/3} mutual tangencies.

Here’s the main idea. If a curve C starts life in A^2, you can lift it to a curve C’ in A^3 by sending each point (x,y) to (x,y,z) where z is the slope of C at (x,y); of course, if multiple branches of the curve go through (x,y), you are going to have multiple points in C’ over (x,y). So C’ is isomorphic to C at the smooth points of C, but something’s happening at the singularities of C; basically, you’ve blown up! And when you blow up a tacnode, you get a regular node — the two branches of C through (x,y) have the same slope there, so they remain in contact even in C’.

Now you have a bunch of bounded degree curves in A^3 which have an unexpectedly large amount of intersection; at this point you’re right in the mainstream of incidence geometry, where incidences between points and curves in 3-space are exactly the kind of thing people are now pretty good at bounding. And bound them we do.

Interesting to let one’s mind wander over this stuff. Say you have n curves of bounded degree. So yes, there are roughly n^2 intersection points — generically, these will be distinct nodes, but you can ask how non-generic can the intersection be? You have a partition of const*n^2 coming from the multiplicity of intersection points, and you can ask what that partition is allowed to look like. For instance, how much of the “mass” can come from points where the multiplicity of intersection is at least r? Things like that.

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