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Assume we have a free ring $S$ of rank 2 and $K= S \otimes Q$. Now if we define an equivalent relation similar to the case $S=O_{K}$ then is it true that the $S$-ideal classes form a group ?

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If I understand you correctly, then no. Take $S = \mathbb{Z}[\sqrt{8}]$. Let $I = \langle 2 \rangle$ and let $J = \langle 2, \sqrt{8} \rangle$. Then $I \neq J$, but $IJ = J^2$. The integral closure condition shows up in number theory precisely to avoid cases like this. – David Speyer Sep 25 '11 at 15:09
One can restrict to invertible ideals, and in that case, one still obtains I group. But that's a bit like cheating... – felix Sep 25 '11 at 15:22
That's not cheating, it's how you define ray class groups. – Franz Lemmermeyer Sep 25 '11 at 19:25
Worst title ever! – Kevin O'Bryant Sep 26 '11 at 1:09
To emphasize the point raised by David, every proper subring $S$ of the integers of a quadratic field, besides the subring ${\mathbf Z}$, is free of rank 2 as a ${\mathbf Z}$-module and contains a non-invertible ideal, so the $S$-ideal classes are not a group under multiplication. – KConrad Sep 26 '11 at 12:06

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