# What is the kernel of the evaluation homomorphism?

I'm studying Sharp's Steps in Commutative Algebra, and I need a hint how to proceed with this exercise in the page 26:

First of all, I didn't understand even the notation, what did the author mean by $(X_1-\alpha_1,...,X_n-\alpha_n)$?

Thanks a lot.

• Definition 2.15 in my copy describes that notation as an alternative to $(X_1-a_1)K+\dots+(X_n-a_n)K$ where $K$ is the given polynomial ring. – Karl Kronenfeld Jun 8 '13 at 2:21
• For the proof you can change coordinates so that you can assume WLOG that $\alpha_1 = ... = \alpha_n = 0$. – Qiaochu Yuan Jun 8 '13 at 2:29
• I believe $(X_1-\alpha_1,\dotsc,X_n-\alpha_n)$ is standard notation for the ideal generated by the $X_i-\alpha_i$, which is the set user1 describes. You might also see it written $\langle X_1-\alpha_1,\dotsc,X_n-\alpha_n\rangle$. – Josh Chen Jun 8 '13 at 2:36

Basically the problem can be translated as "Show that if a polynomial in $n$ variables has $(\alpha_1,\ldots,\alpha_n)$ as a root, then it is of the form $f_1\cdot(x_1-\alpha_1)+\ldots +f_n\cdot (x_n-\alpha_n)$ for some polynomials $f_1,\ldots,f_n$.
• I solved for a polynomial of 1 variable, I'm having troubles to prove this statement for more than 1 variable: $p\in Kerf\implies p\in (X_1-\alpha_1,...,X_n-\alpha_n)$, where f is the evaluation homomorphism. I've already done the converse. – user42912 Jun 8 '13 at 3:08
Hint: By the division algorithm, $\, f = (x_1\! -\! a_1) q_1 + r_1,\$ for $\ r_1\in R[x_2,\ldots,x_n].\,$ By induction, $\,r_1 = (x_2\!-\!a_2)q_2 +\cdots + (x_n\!-\!a_n)q_n + r_n,\,$ for $\,r_n\in R.\$ Evaluating at $\,x_i = a_i\,$ yields $\,f(a_i) = 0\iff r_n = 0\iff f\in (x_1\!-\!a_1,\ldots,x_n\!-\!a_n)$