[Math] The Euler Characteristic of $\mathbf RP^2$ is a Fraction.

algebraic-topologycovering-spaceshomology-cohomology

Problem 22 in Section 2.2 in Hatcher's Algebraic Topology reads

For $X$ a finite CW complex and $p:\tilde X\to X$ an $n$-sheeted covering space, show that $\chi(\tilde X)=n\chi(X)$.

Here $\chi$ denotes the Euler characteristic

Now we have a $2$-sheeted covering $p:S^2\to \mathbf RP^2$. Applying the result of the problem above, we get $\chi(S^2)=2\chi(\mathbf RP^2)$. Now $S^2$ can be given a CW structure having one $0$-cell and one $2$-cell. Thus $\chi(S^2)=3$. This means that $\chi(\mathbf RP^2)=3/2$.

I know that I must be doing something stupid. But I have been stuck at this. Can someone please point out my error. Thank you.

Best Answer

To close the question, let me write my comment as an answer. $\chi(S^2)=2$, not $3$ as can be seen by computing the alternating sum of the Betti numbers.

Alternatively, one may consider either of the two usual cell decompositions of the sphere - either the "one $0$-cell, one $2$-cell" or "one $0$-cell, one $1$-cell, two $2$-cells" description of the sphere - to see that $\chi(S^2) = 1-0+1 = 1-1+2 = 2$.

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