# What are some theorems that are intuitively true but very difficult to prove?

Asked by PhiNotPi (12647) October 19th, 2012

Today I came across the Jordan curve theorem, which seems as if it should be very easy to prove.

The basic statement is this: given any closed, non-self-intersecting loop, it is impossible to draw a path from a point outside the loop to a point inside the loop without crossing the edge of the loop. Here is an example.

This fact went unproven for a very long time, and the current proofs are incredibly long. The reason is that loops can be much more complex than anything that can be drawn by humans. They can contain fractal edges and be nowhere-differentiable.

Here is an example of a proof for the Jordan curve theorem. It is a computer-assisted proof, so it cannot be understood unless you know the language; however, its length demonstrates how hard it is to prove the Jordan curve theorem.

My question is: what are some other examples of mathematical facts that are very intuitively true, but which are very difficult to actually prove?

Observing members: 0 Composing members: 0

Fermat’s conjecture?

Kayak8 (16433)

@Kayak8 Maybe not quite as intuitive, but that sure did take a while to prove (358 years).

PhiNotPi (12647)

The extreme value theorem is another example.

flutherother (27083)

1+1=2 is surprisingly hard to prove.
http://blog.plover.com/math/PM.html

28lorelei (2509)