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2h
answered How do I decide what problems and how many problems to do when I try to self study?
1d
comment How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
Dear @Fredrik, I have now explained that notation in an Edit to the answer.
1d
revised How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
added 558 characters in body
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
comment How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
Dear @zcn, you are probably right but I read the question too superficially and thought the OP just wanted a solution avoiding the Segre embedding, which I gave. I have give other, completely different proofs. Some are probably too advanced for a beginner but I had fun thinking them up for people of your caliber :-)
1d
revised How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
added 199 characters in body
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
comment How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
Good answer, but then one has to explain why a morphism $\mathbb{P}^2\to\mathbb{P}^1$ must be constant.
1d
answered How to show $P^1\times P^1$ (as projective variety by Segre embedding)is not isomorphic to $P^2$?
1d
answered Example of $x$ being adherent point but not accumulation point?
Oct
20
awarded  Necromancer
Oct
20
comment Is product of prime ideals prime?
Thanks for your quick answer and the link, @user26857.
Oct
20
comment Is product of prime ideals prime?
Is there a general principle guiding you to send successively the variables $x_2,x_3,x_4$ and then $x_2,x_4$ to zero?
Oct
20
comment three axis in $\mathbb{A}^3$ can't be defined by two functions
Two solutions: here math.stackexchange.com/a/92518/3217 and here math.stackexchange.com/a/510997/3217
Oct
19
comment Finding a line $L\subset V(y-xz)\subset\mathbb A^3_k$
More advanced users will recognize in $V(y-xz)$ one of the two standard affine open subsets covering the blow-up of the affine plane at the origin.
Oct
19
comment Finding a line $L\subset V(y-xz)\subset\mathbb A^3_k$
Your second sentence is not quite correct: the inverse image of $M$ is only one line if $a=1, b=0$. But your final sentence indeed correctly solves the problem:+1 .