$\mathbb Z[\sqrt d]$ is not a field.

Question

How shall I show that $\mathbb Z[\sqrt d]=\{a+b\sqrt d~~\big|a,b\in \mathbb Z\}$ is not a field?

It is an integral domain, so the thing it lacks maybe is every element does not have a multiplicative inverse. How to show this?

Answer 1

WLOG, $\sqrt{d} \notin \mathbb{Z}$ (otheriwse $\mathbb{Z}[\sqrt{d}] = \mathbb{Z}$ is not a field). Suppose $\sqrt{d}(a + b\sqrt{d}) = 1$. Then $a\sqrt{d} = 1 - bd$ is an integer which is only possible if $a = 0$. So $bd = 1 \implies d = 1$ which is a contradiction.

Answer 2

Is $2$ invertible in this ring? That is, $1/2\in \mathbb Z[\sqrt d]$?