# Families of ideals with a given initial ideal

Assume a fintie set of monomials is given. Is there a way to find the family of ideals whose initial ideal (say w.r.t revlex order) is generated by that finite set? I'll appreciate any partial answer, reference or suggestions.

## 1 Answer

It's kinda gross, but it can be done.

To each monomial, add a generic linear combination of all smaller monomials (w.r.t. your term order).

Now insist that what you have is a Gr\"obner basis. How do you do this? Apply the reduction algorithm to each S-polynomial, until you get stuck. Then assert that the result is zero. This puts a multitude of algebraic conditions on the coefficients in your generic linear combinations.

I hope it's obvious that the resulting set thus constructed, a Bia\l ynicki-Birula stratum on the Hilbert scheme, should be termed the "Gr\"obner basin".

• Thank you for your answer. Theoretically it is true But the problem ist I don't know how to be sure that the combination is a "generic" one in a concrete given situation. However I know that this problem should not be very easily accessible too, since the consequences are very surprising! Commented Apr 8, 2015 at 7:12
• Sorry, all I mean is that you should put in a coefficient for every possible term, then use the above to constrain the coefficients. This answer makes no assumption of genericity; it really does find every ideal with this initial ideal. Commented Apr 8, 2015 at 10:55