# homework problem about the projective real space

Sorry for ask this problem, but I am very complicated with this problem :/ . My course it´s of topology, the teacher said that we only need the definition of the quotient topology and of $$P_R^2$$ but I do not feel comfortable working with the projective plane, still not, and we've never seen anything more about this. But I can use the theorems of chapter 1 of massey on triangulations, about the Euler characteristic is independent of the triangulation, and so on. The problem is this :

First let's define a line as a set of the form $$L_{a,b,c} = \left\{ {\left[ {x,y,z} \right] \in P_R^2 :ax + by + cz = 0} \right\}$$ Now let's define an array of lines, as a finite collection of lines $$\left\{ {L_i } \right\}_{i = 1}^n$$ such that $$\bigcap\limits_{i = 1}^n {L_i }$$ it´s empty . A paving of polygons of $${P_R^2 }$$ R is the same definition of "on triangles", but now you can consider any polygons. Prove that the Euler characteristic of $${P_R^2 }$$ R can be calculated with any paving of polygons \begin{align*} & v - \ell + p \\ v &= \text{number of vertices}\\ \ell &= \text{number of sides}\\ p &= \text{number of polygons} \end{align*} How can i prove this? I can use the theorem that the number is independent of the triangulation. It should not be at all difficult, but I'm a little unsure )= in the formalism. I think that I must triangulate each polygon and, and in someway count have v-l + p using the triangles sorry for the stupid question

-
I think your last comment is on the right track. Pick a polygon and triangulate it. How do $v$, $l$, and $p$ change? – Adam Saltz Oct 4 '11 at 2:55
I did it! thanks for all – August Oct 5 '11 at 1:51