The Stacks project

Example 15.65.3. Let $k$ be a field and let $R$ be the ring of dual numbers over $k$, i.e., $R = k[x]/(x^2)$. Denote $\epsilon \in R$ the class of $x$. Let $M = R/(\epsilon )$. Then $M$ is quasi-isomorphic to the complex

\[ R \xrightarrow {\epsilon } R \xrightarrow {\epsilon } R \to \ldots \]

but $M$ does not have finite projective dimension as defined in Algebra, Definition 10.108.2. This explains why we consider bounded (in both directions) complexes of projective modules in our definition of bounded projective dimension of objects of $D(R)$.


Comments (0)


Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.




In order to prevent bots from posting comments, we would like you to prove that you are human. You can do this by filling in the name of the current tag in the following input field. As a reminder, this is tag 0A5Q. Beware of the difference between the letter 'O' and the digit '0'.