Minimal degree of polynomial vanishing on the variety of small degree. - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-21T07:56:24Z http://mathoverflow.net/feeds/question/103047 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/103047/minimal-degree-of-polynomial-vanishing-on-the-variety-of-small-degree Minimal degree of polynomial vanishing on the variety of small degree. Klim Efremenko 2012-07-24T20:24:22Z 2012-07-30T20:38:09Z <p>My question is assume that we know that the degree of some irreducible variety is small does it possible to conclude that there exists polynomial of small degree vanishing on this variety. </p> <p>Let us make the question more concrete: Let \$V\subset A^{2n}\$ be an irreducible algebraic variety of dimension \$n\$ and degree \$d\$ (say \$2^n\$). Let \$I \triangleleft \mathbb{C}[x_1,\ldots, x_{2n}]\$ be an ideal of all polynomials vanishing on \$I\$. My question is what is the best upper bound on the minimal degree of polynomials in \$I\$. </p> <p>The best upper bound that I know is \$2^n\$, while I think that it is not the true. For example if \$V\$ is a complete intersection variety then \$I\$ must have polynomial of degree \$2\$. </p> http://mathoverflow.net/questions/103047/minimal-degree-of-polynomial-vanishing-on-the-variety-of-small-degree/103526#103526 Answer by Jesko Hüttenhain for Minimal degree of polynomial vanishing on the variety of small degree. Jesko Hüttenhain 2012-07-30T16:47:13Z 2012-07-30T16:47:13Z <p>The minimal degree of a polynomial that vanishes on \$V\$ is the minimal \$m\$ such that \$h_I(m)\ne 0\$, where \$h_I\$ denotes the Hilbert function of the variety \$V\$. Hence, you are interested in upper (and lower?) bounds on the Hilbert function of certain varieties. There are two papers I know that deal with such bounds:</p> <ul> <li><a href="http://atlas.mat.ub.es/personals/sombra/publications/hilbert/hilbert5.pdf" rel="nofollow">http://atlas.mat.ub.es/personals/sombra/publications/hilbert/hilbert5.pdf</a></li> <li><a href="http://www.ams.org/journals/spmj/2008-19-06/S1061-0022-08-01031-5/S1061-0022-08-01031-5.pdf" rel="nofollow">http://www.ams.org/journals/spmj/2008-19-06/S1061-0022-08-01031-5/S1061-0022-08-01031-5.pdf</a></li> </ul> http://mathoverflow.net/questions/103047/minimal-degree-of-polynomial-vanishing-on-the-variety-of-small-degree/103547#103547 Answer by Jérôme Poineau for Minimal degree of polynomial vanishing on the variety of small degree. Jérôme Poineau 2012-07-30T20:38:09Z 2012-07-30T20:38:09Z <p>You can have a look at the article "Direct methods for primary decomposition" by Eisenbud, Huneke and Vasconselos (Inventiones 110, 1992), available on Eisenbud's webpage. In proposition 3.5, they prove (and say it is long known) that a homogeneous equidimensional ideal \$I\$ of degree \$d\$ is generated up to radical by forms of degree at most \$d\$. </p> <p>See also the remark following the proposition for an example showing it is sharp: \$V\$ is the union of \$d\$ skew lines that all meet a common line \$L\$. They also mention a conjecture in the case where \$I\$ is prime: up to a radical it should be generated by form of degree at most \$d-\$ codim \$I + 1\$. </p>