Search All of the Math Forum:
Views expressed in these public forums are not endorsed by
Drexel University or The Math Forum.
|
|
Mike
Posts:
63
Registered:
6/2/06
|
|
Re: Nine Lemma
Posted:
Jun 28, 2006 2:24 AM
|
|
John Baez wrote:
> In article <e5diqt$r21$1@news.ks.uiuc.edu>,
> John Baez <baez@math.removethis.ucr.andthis.edu> wrote:
>
> >What's a famous, catchy, exciting or illuminating application
> >of the Nine Lemma in homological algebra? The Wikipedia article
> >on this subject needs some help, and someone asked me this...
>
> So nobody knows any use for the Nine Lemma? Interesting....
An overstatement to say that nobody knows any use for it. To my
knowledge the 9 lemma is never strictly NECESSARY. But it is handy to
have around. It conceptually illuminates homological algebra (assuming
that one has a brain as strange as mine that feels illuminated by the
result). One one has the9 lemma one can give tidy proofs of relative
Mayer-Vietoris and excision as others in the thread have pointed out.
Actually, I have always thought that the hexagonal lemma is
overrated as to usefulness. Opinions everybody?
Mike
|
|
|
|
