Computer calculations show that the class group of $F = \mathbb{Q}[x]/(x^2 - x + 18) \simeq \mathbb{Q}(\sqrt{-71})$ is $C_F = \mathbb{Z}/7\mathbb{Z}$. Here is the table.
The ideal class group is defined as the fractional ideals modulo the principal (fractional) ideals. It measures the extent to which unique factorization fails. It would be instructive to see the prime ideals which factor in this manner.
In the article on fractional ideals there's an exact sequence: $$ 0 \to \mathcal{O}_K^\times \to F^\times \to I_F \to C_F \to 0 $$ so if I enter the values that I know so far: $$ 0 \to \Big[\mathbb{Z}[\sqrt{-71}]^\times \simeq \{ +1, -1\}^\times \Big] \to \mathbb{Q}(\sqrt{-71})^\times \to I_F \to \mathbb{Z}/7\mathbb{Z} \to 0 $$ From the abstract algebra point of view, I don't know how a cycle group emerges here. $\mathbb{Q}(\sqrt{-71})^\times$ is an infinitely generated abelian group under the operation (and also $\mathbb{Q}^\times$). I'd really like to see how to use the exact sequence.
The word "ideal" is used here differently:
- a fractional ideal of $\mathcal{O}_F$ is an $\mathcal{O}_F$-module $I \subseteq F$ (of "fractions") with $rI \subseteq \mathcal{O}_F$ for some $r \in F$.
Another question about the "ring theory" involved here. Using different example: