Real plane curve

In mathematics, a real plane curve is usually a real algebraic curve defined in the real projective plane.

Ovals

The field of real numbers is not algebraically closed, the geometry of even a plane curve C in the real projective plane. Assuming no singular points, the real points of C form a number of ovals, in other words submanifolds that are topologically circles. The real projective plane has a fundamental group that is a cyclic group with two elements. Such an oval may represent either group element; in other words we may or may not be able to contract it down in the plane. Taking out the line at infinity L, any oval that stays in the finite part of the affine plane will be contractible, and so represent the identity element of the fundamental group; the other type of oval must therefore intersect L.

There is still the question of how the various ovals are nested. This was the topic of Hilbert's sixteenth problem. See Harnack's curve theorem for a classical result.

gollark: > what's a pacman-like CLI?Arch Linux (btw I use that) has a neat package manager called `pacman`.> what counts as package updating support?Updating packages without breaking things horribly, including not overwriting user-edited (config) files.> and library interface as in an API you can use from scripts?Precisely.
gollark: Oh, and a library interface.
gollark: Well, I would want a pacman-like CLI, probably configurable repos, multiple files in a package, good package updating support, and... other stuff?
gollark: If CC had symlinks, which it doesn't without a ton of FS hackery, you could make a busybox-type thing.
gollark: I might actually do that for PotatOS Hexahedron™, the upcoming probably never™ lightweight potatOS version.

See also

References
  • "Plane real algebraic curve", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
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