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Let $Y=\{(t,t^2,t^3)\mid t\in k\}$ be the twisted cubic curve. I'm trying to prove this curve is a variety, i.e., it's irreducible and affine algebraic set.

The easier part is to prove the twisted cubic curve is an affine algebraic set $(Y=Z(x^2-y,x^3-z))$.

I don't know how to prove that $Y$ is irreducible, I'm trying to prove that $(x^2-y,x^3-z)$ is prime, I think if I do this I proved what I want, but I found this hard to prove.

I need help to finish this question.

Thanks a lot.

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    $\begingroup$ Please use the word "prove" appropriately. In what you call "the easy part", you didn't prove anything; you applied a trivial definition. $\endgroup$ Nov 15, 2013 at 20:03
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    $\begingroup$ Show that the quotient is an integral domain by exhibiting an explicit isomorphism with the polynomial ring in one variable. $\endgroup$
    – Zhen Lin
    Nov 15, 2013 at 20:35
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    $\begingroup$ @ZhenLin Do you mean $k[x,y,z]/(x^2-y,x^3-z)\cong k[x,x^2,x^3]\cong k[t]$? $\endgroup$
    – user75086
    Nov 15, 2013 at 21:23
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    $\begingroup$ How do we know $Y = Z(x^2-y, x^3-z)$? $\endgroup$ Aug 24, 2020 at 17:52

4 Answers 4

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Let's first prove that ideal $I:=(x^2-y, x^3-z)$ is prime. Suppose $f\cdot g \in I$. Using obvious isomorphisms $k[x,y,z] \cong (k[x,y])[z]$, $k[x,y] \cong (k[x])[y]$ and division algorithm, we have $$ f(x,y,z)=(x^3-z)f_1(x,y,z) + (x^2-y)f_2(x,y) + f_3(x), $$ $$ g(x,y,z)=(x^3-z)g_1(x,y,z) + (x^2-y)g_2(x,y) + g_3(x). $$ Now we have $f_3(x) \cdot g_3(x) \in I$, therefore $$f_3(x) \cdot g_3(x)=(x^3-z)h_1(x,y,z) + (x^2-y)h_2(x,y,z).$$ Insert $(x,y,z)=(t,t^2,t^3)$ and get $f_3(t) \cdot g_3(t) =0$ for all $t \in k$. If $k$ is algebraically closed (therefore infinite), we have $f_3 \cdot g_3 = 0$, so $f_3 = 0$ or $g_3= 0$. Then $f \in I$ of $g \in I$, so $I$ is prime (and therefore radical). We have $I(Y)=I(V(I)) = \operatorname{Rad}(I) = I$, which is prime. So $Y$ is irreducible.

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    $\begingroup$ There is no need to assume that $k$ is algebraically closed (and the OP didn't do it): in the last polynomial relation plug in $y=x^2$ and $z=x^3$ to find $f_3=0$ or $g_3=0$. $\endgroup$
    – user89712
    Nov 23, 2013 at 9:44
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    $\begingroup$ Element calculations make everything complicated ... $I$ is prime because obviously $k[x,y,z]/(I) = k[x]$. $\endgroup$ Nov 23, 2013 at 16:40
  • $\begingroup$ Thank you for you answer, I have a question, why can you use division algorithm? $\endgroup$
    – user75086
    Dec 3, 2013 at 11:04
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    $\begingroup$ Not necessarily. It is enough that $k[x,y]$ is a commutative ring, and that leading coefficient of $x^3-z$ is an unit in that ring. The leading coefficient is $-1$ (in variable $z$ of course). Proof is the same as in the case of fields. $\endgroup$ Dec 4, 2013 at 9:01
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    $\begingroup$ No, no calculation is needed. It is trivial to check that both sides represent the same functor, hence they are isomorphic (Yoneda). $\endgroup$ Dec 5, 2013 at 0:53
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There is an obvious isomorphism $Y \cong \mathbb{A}^1$. This proves everything else.

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  • $\begingroup$ How do you know which polynomials to choose to define the variety? $\endgroup$ Mar 31, 2021 at 22:06
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Expand a little on Martin's answer.

Define $f:X \to Y$ as $$f(x) = (x, x^2, x^3)$$ Define $g:Y \to X$ as $$g(x,y,z) = x$$

Then $f^{-1} = g$, $f$ is an isomorphism between $A^1$ and Y.

Now by p.g.29 of Shafarecich's Basic algebraic Geometry 1, there exists an isomorphism between coordinate rings of $A^1$ and $ Y$, which means $$k[x] \cong k[x,y,z]/((x^2-y),x^3-z)$$

Since k[x] is an integral domain, $k[x,y,z]/((x^2-y),x^3-z))$ is an integral domain, which means that $((x^2-y),x^3-z))$ is irreducible.

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Here is a way to show that $Y$ is irreducible with little computation: note that since $Y$ is closed (as the set of zeros of two polynomials) we have $Y=Z(I(Y))$, where $I(Y)$ is the ideal of $Y$. The only thing to check is that $I(Y)$, is prime. But it is easy to see that $I(Y)$ is the kernel of a surjective ring homomorphism $\phi:k[x,y,z]\to k[t]$, namely the homomorphism such that $\phi(x)=t,\phi(y)=t^2,\phi(z)=t^3$. Hence the ring $A(Y)=k[x,y,z]/I(Y)$ is isomorphic to $k[t]$, which is integral, hence $I(Y)$ is prime, and $Y$ is an affine variety isomorphic to the affine line. Q.E.D.

Of course the ideal $I(Y)$ is the radical of $(x^2-y,x^3-z)$, and the above arguments show that in fact $I(Y)=(x^2-y,x^3-z)$, but this is not needed.

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