## An incorrect proof of the Jordan Curve Theorem – can you find what’s wrong with it?

Every closed curve in an infinite flat plane divides it into a bounded part and an unbounded part (inside and and outside if you’re not particular). This is so screamingly obvious, that for a long time no one thought it needed proof. Bolzano changed all that about 200 years ago, but a proof was not forthcoming until Jordan gave a proof (thought by most to be defective) in 1887.

The proof is long and subtle. The one I’ve read uses the Brouwer fixed point theorem, which itself uses the fact that fundamental group of a circle is infinite cyclic (and that’s just for openers). You begin to get the idea.

Imagine the 4 points (1,1),(1,-1),(-1,1) and (-1,1) the vertices of a square centered at ( 0, 0 ). Now connect the vertices by straight lines (no diagonals) and you have the border of the square (call it R).

We’re already several pages into the proof, when the author makes the statement that R “splits R^2 (the plane) into two components.”

It seemed to me that this is exactly what the Jordan Curve theorem is trying to prove. I wrote the author saying ‘why not claim victory and go home?.

I got the following back

“It is obvious that the ‘interior’ of a rectangle R is path connected. It is
only a bit less obvious – but still very easy – to show that the ‘exterior’
of R is also connected. The rest of the claim is to show that every path
alpha from a point alpha(O)=P inside the rectangle R to a point alpha(1)=Q
out of it must cross the boundary of R. The set of numbers S={alpha(i) :
alpha(k) is in interior(R) for every k≤i} is not empty (0 is there), and it
is bounded from above by 1. So j=supS exists. Then, since the exterior and
the interior of R are open, j must be on the boundary of R. So, the interior
and the exterior are separate components of R^2 \ R. So, there are two of
them.”

Well the rectangle is topologically equivalent (homeomorphic) to a circle.

So why isn’t this enough?  It isn’t ! !

Answer to follow in the next post. Here’s the link — go to the end of the post — https://luysii.wordpress.com/2015/11/10/are-you-sure-you-know-everything-your-protein-is-up-to/