# Smooth immersion(?) of graphs into the plane

Sorry if the terminology's wrong, I don't know differential topology. Also, this is more of a brain-teaser than a bona fide research question, but it's hopefully a "real mathematician"-level brain-teaser.

So the crossing number inequality gives a lower bound for how many intersections you have to have if you draw a graph with enough edges in the plane. The thing is, the crossing number inequality counts intersections "with multiplicity." It's pretty easy to see that if you don't count intersections with multiplicity, you can draw any finite graph in the plane with only one "crossing point."

However, while there's an easy explicit description of such a drawing, the edges aren't smooth (or even first-differentiable!) So my question is in two parts:

1. Is there a drawing of K_n in the plane such that there is exactly one point where edges can intersect, and all edges are smooth embeddings of the (open) unit interval in R^2?

2. Is there an explicit description of such a drawing? (E.g., can you write the edges as real algebraic curves?)

(Note to moderators: you might want to tag this as "recreational" or "brain-teaser" or something of that sort; I don't have 250 reputation and so can't. :-/)

• This seems like a perfectly reasonable question to me. It's not particularly "recreational." I wouldn't worry so much about defending your question. – Noah Snyder Oct 18 '09 at 23:00
• @Noah: I think what I meant by "recreational" was that it didn't arise so much from Serious Thinking about a Serious Problem as from saying "oh, hey, this problem's cool!" – Harrison Brown Oct 18 '09 at 23:01
• But lots of math is done because someone thinks "oh, hey, this problem is cool" – Theo Johnson-Freyd Oct 19 '09 at 5:05