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Apr
24
answered Cohen-Macaulay rings and Normal rings
Feb
9
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
Feb
9
awarded  Commentator
Feb
9
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.
Feb
9
awarded  Yearling
Feb
7
answered pullback of global sections with respect to an automorphism of schemes
Feb
5
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!
Feb
5
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?
Feb
5
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?
Feb
5
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
Feb
5
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.
Feb
5
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?
Feb
4
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.
Feb
4
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.
Feb
4
answered How to write a divisor: Exercise n 20 pag 285 from Geometry of algebraic curves by Arbarello, Cornalba, Griffiths, Harris
Sep
10
awarded  Editor
Sep
10
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
Sep
10
awarded  Supporter
Sep
10
answered Does $\mathbb{Z}_5(\sqrt[3]{3})$ make sense? Or, can we always extend a field by a root of a reducible polynomial?
Sep
8
answered When is the product of $n$ subgroups a subgroup?