Verifying that the ideal $(x^3-y^2)$ is prime 
How to prove that the ideal $I=(x^3-y^2)$ in $k[x,y]$ is prime? 

I have constructed a map from $k[x,y]$ to $k[t]$, which maps $x$ to $t^2$, and $y$ to $t^3$. Then, I want to show that the kernel is exactly $I$. But the difficulty is that how to prove the kernel is contained in $I$. 
Could you help me? Thanks a lot!
 A: Take an element $f(x,y)\in Ker f$ i.e. Now, consider $f(x,y)$ as a polynomial in variable $x$ and coefficients coming from $k[y]$. If you divide $f(x,y)$ by $(x^3-y^2)$ we will get $f(x,y)=g(x,y)(x^3-y^2)+r(x,y)$ where $r(x,y)\in (k[y])[x]$ and degree of $r(x,y)$ is less than three. But then $f(t^2,t^3)=0$ implies $r(t^2,t^3)=0$. But $r(t^2,t^3)$ cannot be zero beacuase  $r(x,y)$ is a  polynomial of degree less than three in variable $x$ with coefficients in $k[y.]$
Explanation of "But $r(t^2,t^3)$ cannot be zero beacuase  $r(x,y)$ is a  polynomial of degree less than three in variable $x$ with coefficients in $k[y]$": 
$r(x,y)=h_1(y).x^2+h_2(y).x+h_3(y)$. So $r(t^2,t^3)=h_1(t^3).({t^2})^2+h_2(t^3).t^2+h_3(t^3)$ . Let the highest power of $h_1(t^3).({t^2})^2$ is $t^{3m+4}$ where m is the degree of $h_1(t)$. Similarly the highest power of$h_2(t^3).t^2$ is $t^{3n+2}$  where n is the degree of $h_2(t)$. and the highest power of $h_3(t^{3})$ is $t^{3k}$ where  k is the degree of $h_3(t)$. 
If $r(t^2,t^3) $ has to be zero then $3m+4=3n+2=3k$ for some $m,n,k \in \mathbb Z$
which is same as saying $3m+1=3n+2=3k$ for some $m,n,k \in \mathbb Z$. Whioch is impossible. So $r(t^2,t^3)=0$ And $f(x,y)\in Ker$
A: It suffices to prove that $x^3 - y^2$ is irreducible in $k[x, y]$.
Hence it suffices to prove that it is irreducible in $k(y)[x]$.
This is clear because it has no root in $k(y)$.
A: You want to prove the following: if $f\in k[x,y]$ and $f(t^2,t^3)=0$ then $f\in(x^3-y^2)$. 
Let's divide $f(x,y)$ to $y^2-x^3$ by considering both as polynomials in $y$ with coefficients in $k[x]$. (This can be done since $y^2-x^3$ is a monic polynomial in $y$.) We have $$f(x,y)=(y^2-x^3)g(x,y)+r(x,y)$$ with $\deg_yr<2$, so we can write $r(x,y)=r_1(x)+r_2(x)y$. Since $f(t^2,t^3)=0$ we get $r_1(t^2)+r_2(t^2)t^3=0$. Now note that the degree in $t$ of $r_1(t^2)$ is even, while the degree of $r_2(t^2)t^3$ is odd. Can you conclude from this that $r_1=r_2=0$?
