I know that there is a sample from TI, that shows that it's possible.
But I couldn't really understand that file riemann.lua

Does anyone know how I can I do it, or know a better example for understanding.this?

I just need a calculation sheet, so the user can input a function e.g. define f2(abobora)=1/abobora!

Yes I have used it, and I just figured it out, after pasting some lua code

I was looking for a way to split the screen using lua code, and I read that code at least 12 times,
looking where it was done 

I was looking for a way to split the screen using lua code, and I read that code at least 12 times,
looking where it was done 

No, there is no way you can split the screen in pure-lua. a lua script is embedded in a "widget", and when you split the screen, it just creates a place for another widget, where everything (including another lua script) can go.
So, splitting the screen has nothing to do with the lua code itself.

If you're looking to know how some variables are shared between these widgets, take a look at the 'var' APIs

just do :

if teste then
....
end