Lemma 15.128.1 (Eilenberg's lemma). If $P \oplus Q \cong F$ with $F$ a nonfinitely generated free module, then $P \oplus F \cong F$.

** Eilenberg swindle **

[Eilenberg's lemma, Bass]

In [Bass] we find: “...is an elegant little swindle, observed several years ago by Eilenberg, and which might well have sprung from the brow of Barry Mazur.”

**Proof.**

\[ F \cong F \oplus F \oplus \ldots \cong P \oplus Q \oplus P \oplus Q \oplus \ldots \cong P \oplus F \oplus F \oplus \ldots \cong P \oplus F \]

$\square$
## 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Your email address will not be published.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.

## Comments (0)