Ideals of etale structure sheaves - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-18T20:41:16Z http://mathoverflow.net/feeds/question/102463 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/102463/ideals-of-etale-structure-sheaves Ideals of etale structure sheaves Colin McLarty 2012-07-17T15:58:40Z 2012-07-26T01:28:10Z <p>Is it known whether or not every sheaf of ideals of the etale structure sheaf of a Noetherian scheme is generated by finitely many of its sections? Of course it is trivially true for some widely used special cases. But is it known one way or the other, in this generality?</p> http://mathoverflow.net/questions/102463/ideals-of-etale-structure-sheaves/103142#103142 Answer by Colin McLarty for Ideals of etale structure sheaves Colin McLarty 2012-07-26T01:21:26Z 2012-07-26T01:28:10Z <p>I must apologize for posting a false answer. in writing up a proof i discovered a gap which grew to a counterexample.</p> <p>In fact not every sheaf of ideals of an etale structure sheaf is finitely generated. I have added a counterexample to the end of my ArXiv paper on cohomology in second order arithmetic arXiv:1207.0276v2. Intuitively, an etale ideal can hold information about arbitrarily high degree extensions which is not reducible to information about any fixed degree so the ideal is not finitely generated. </p> <p>The counterexample shows that given any non-zero element \$x\$ of an algebraically closed field \$k\$ a single etale sheaf of ideals on the punctuated line \$k\0\$ can pick one \$2^n\$-th root of \$x\$ for every \$n\$. The example has \$x=1\$ but rescaling \$k\0\$ takes 1 to any other \$a\neq 0\$.</p>