most recent 30 from http://mathoverflow.net 2013-05-24T00:43:56Z http://mathoverflow.net/feeds/question/50432 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/50432/a-noetherian-proof-of-zariskis-main-theorem Akhil Mathew 2010-12-26T15:38:14Z 2010-12-27T17:26:24Z

<p>Recall that Zariski's Main Theorem states that if $f: X \to Y$ is a quasi-finite, separated, and finitely presented morphism into a quasi-compact separated scheme $Y$, then there is a factorization of $f$ into an open immersion followed by a finite morphism. In EGA IV-8, this is proved by reducing to the case of $Y$ the $\mathrm{Spec}$ of a noetherian ring by a finite presentation argument (the general machinery of which is developed in the prior part of that section), then reducing to the case of a local noetherian excellent ring (by again using the finite presentation argument, since by this machinery proving things about the local scheme $\mathrm{Spec}(\mathcal{O}_y)$ is the same as proving things in a neighborhood), and finally by completing and proving the result for $Y$ the spectrum of a complete local noetherian ring, after which it is basically commutative algebra. </p> <p>This argument is very pretty, but I am curious if there is a more elementary approach in the special case of $Y$ noetherian, or even in the classical case of schemes of finite type over a field (that avoids the general machinery of finite presentation arguments and the descent of properties of morphisms under faithfully flat base-change). Namely, I am curious whether there is an argument that uses less fancy machinery, and could be phrased in the language of varieties. Is there one?</p>

http://mathoverflow.net/questions/50432/a-noetherian-proof-of-zariskis-main-theorem/50435#50435 arsmath 2010-12-26T16:18:36Z 2010-12-26T16:18:36Z

<p>There's a purely algebraic proof in some <a href="http://www.math.lsa.umich.edu/~hochster/615W10/615.pdf" rel="nofollow">lecture notes</a> by Mel Hochster. He explains the translation into the language of varieties, as well.</p>

http://mathoverflow.net/questions/50432/a-noetherian-proof-of-zariskis-main-theorem/50502#50502 Leo Alonso 2010-12-27T17:26:24Z 2010-12-27T17:26:24Z

<p>I would suggest chapter IV of the 1970 book "Anneaux Locaux Henséliens", by Michel Raynaud published in Springer Lecture Notes in Math no. 169. It gives a very general proof, way simpler than the one in EGA IV and, in my opinion, very readable. The proof is based in a paper by Peskine from 1966. The proof in Raynaud's book is complete, as far as I can recall.</p> <p>As a footnote, sometimes noetherian hypothesis do not make arguments simpler, but, of course, this depends on the issue at hand.</p>