Sándor Kovács
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 Feb9 revised How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris deleted 382 characters in body Feb9 awarded Commentator Feb9 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris OK, I think this indeed does it. Feb9 awarded Yearling Feb7 answered pullback of global sections with respect to an automorphism of schemes Feb5 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris If you care to explain that argument regarding the norm we can see whether it works. You could post it as another answer with some more details... Cheers! Feb5 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris Actually, come to think of it, the definition of $a_1$ and $a_2$ makes no difference. If $a_1+a_2=a$, then $-\tau\left((a_1-\frac a2)P_1\right)=(a_2-\frac a2)P_2$ holds regardless, so this could not have been the problem. But then (again): with what exactly are you having a problem? Feb5 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris I've just realized that I did not specify what $a_i$ were. I guess I thought it was obvious, sorry. I edited the answer now. Was that the issue? Feb5 revised How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris added 41 characters in body Feb5 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris that's the right side. :) What exactly is your question? $\tau$ swaps the points $P_1$ and $P_2$, so this equality follows from the previous one. Feb5 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris so where is $a_2$ in that? Feb4 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris there is no $a_2$ on the left part. Which equation are you looking at? // Maybe it is true and maybe your norm calculation is OK, I'm just saying that you are stating facts based on some mystery book. Feb4 comment How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris dario: 1) Yes, I said that this works if $a$ is even. 2) You assumed that $\alpha_*E$ is trivial and hence $\equiv 0$ 3) I'm not convinced by your norm calculation. Feb4 answered How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris Sep10 awarded Editor Sep10 revised Does $\mathbb{Z}_5(\sqrt[3]{3})$ make sense? Or, can we always extend a field by a root of a reducible polynomial? added 70 characters in body Sep10 awarded Supporter Sep10 answered Does $\mathbb{Z}_5(\sqrt[3]{3})$ make sense? Or, can we always extend a field by a root of a reducible polynomial? Sep8 answered When is the product of $n$ subgroups a subgroup? Sep7 comment Acyclic vs Exact (You mean Brown, right?). No. Here is what he says on page 5: "acyclic, i.e., $H(C)=0$".