I'm having trouble determining if the algebraic sets $Z(x^2-y^3)\subset \mathbb{A}^2$ and $Z(y^2-x^3-x^2)\subset\mathbb{A}^2$ are isomorphic over $\mathbb{C}$. My guess is that this boils down to determining if $\mathbb{C}[x,y]/(x^2-y^3)$ is isomorphic to $\mathbb{C}[x,y]/(y^2-x^3-x^2)$ but then again, I'm stuck.
Is $Z(x^2-y^3)$ isomorphic to $Z(y^2-x^3-x^2)$ over the complex numbers?
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abstract-algebra
algebraic-geometry
polynomials
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0Isomorphic as sets? as abelian groups? as rings? – 2012-10-24
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0Isomorphic as affine varieties. – 2012-10-24
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1This is a dublicate of my question http://math.stackexchange.com/questions/128918/cusp-and-node-are-not-isomorphic. There I also give an alternative proof. – 2012-10-24
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0Thanks for pointing it out! – 2012-10-24
1 Answers
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Thinking geometrically, we expect these varieties are not isomorphic, due to the fact that the first is a cusp, while the second is a node. One way to verify this is to consider the tangent cone of each. In the first case, we get $TC_{(0,0)}=V(x^2)$ which is interpreted as the line $x=0$ with multiplicity $2.$ In the second case, we get $TC_{(0,0)}=V(y^2-x^2)=V((y-x)(y+x))$ which is two distinct lines.
Since the tangent cone is an invariant under isomorphism, we see that there is no point on the first variety to correspond with the origin in the second, and vice-versa.
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0Is there a more elementary way to see this? (i.e. without using the tangent cone) – 2012-10-24
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0Dear @Victor, actually, I think you could do essentially the same thing with the tangent space rather than tangent cone, but I actually found this to be easier to calculate! – 2012-10-24
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0@Victor, actually, you should ignore my previous remark; we cannot use the tangent space instead, since both defining polynomials have no linear terms. Thus both curves have tangent space $\cong\Bbb A^2,$ so the tangent cone is quite convenient here. – 2012-10-24
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0I've been trying to do this question too. I'm interested if there is a solution that is basic in the sense that one shows it by showing the rings are not isomorphic or something. – 2012-10-24
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1There are elementary proofs, but they are longer and quite silly compared to geometric arguments. This is really what algebraic geometry is about ... – 2012-10-24