# Zulip Chat Archive

## Stream: maths

### Topic: polynomial.funext

#### Johan Commelin (Sep 21 2020 at 03:32):

Under what conditions on a ring R can I conclude that if a (multivariate) polynomial over R evaluates to zero on all x \in R (or tuples (x_0, .., x_n, ..) in the mv case) that it is the zero polynomial.

- For finite R it's of course false: frobenius
- For infinite integral domains I can prove it.
- After that, you can take products of such rings.
- But I'm not so sure about what happens if R has a nilradical. Can anyone help me out?

#### Bryan Gin-ge Chen (Sep 21 2020 at 03:50):

This MO question seems related: https://mathoverflow.net/questions/160986/rings-for-which-no-polynomial-induces-the-zero-function

#### Johan Commelin (Sep 21 2020 at 04:19):

@Bryan Gin-ge Chen thanks! I'll just stick to infinite integral domains then. Because those answers aren't very clean (-;

#### Kevin Buzzard (Sep 21 2020 at 06:25):

Oh we ran into exactly this question when formalising the basics of algebraic varieties. Our source said k an algebraically closed field, which worked because such things are infinite integral domains :-)

#### Kenny Lau (Sep 21 2020 at 07:04):

Frobenius stands as a counter-example to Fp, but for general finite R = {r1, ..., rn} one can consider (X - r1) ... (X - rn) as a counter-example

#### Johan Commelin (Sep 21 2020 at 07:05):

Sure

#### Johan Commelin (Sep 21 2020 at 11:14):

Last updated: May 09 2021 at 11:09 UTC