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Separately holomorphic functions vanishing on a real box are zero
Statement
Let and let be separately holomorphic: for every and every fixed , the one-variable map is entire on . If there are nondegenerate intervals with on , then on .
Facts & Assumptions
Given: An integer , a separately holomorphic , and nondegenerate intervals (that is, each contains a nonempty open subinterval, so it has more than one point) with on .
Identity theorem: if two functions holomorphic on a complex domain agree on a set having an accumulation point in , then they agree on (Identity theorem for holomorphic functions).
A function is entire when it is complex differentiable on all of , that is, holomorphic on the domain ; a separately holomorphic has every one-variable slice entire by hypothesis (Complex differentiability at a point, the complex derivative, holomorphic functions, and entire functions).
If has nonempty interior and lies in that interior, then is an accumulation point of in : some open ball is contained in , and for every neighbourhood radius , is a point of different from within that neighbourhood. Every coordinate slice of a separately holomorphic function is determined by the values it takes on such a set.
Proof
Base case . Here is an entire function of one variable and vanishes on the nondegenerate interval . Choose in the interior of ; by [F3], is an accumulation point in of the set where and the zero function agree, and both are entire [F2]. The identity theorem [F1] on the domain gives .
Inductive hypothesis and setup. Assume and that the assertion holds for variables. Choose , which is possible because is nondegenerate, and fix an arbitrary ; it remains to show .
The slice in the last variables. The map on is separately holomorphic, because each of its one-variable slices is a slice of with all other coordinates fixed, hence entire by [F2]. It vanishes on the box , whose factors are nondegenerate, so the induction hypothesis of step 1.2 applies and gives .
Vanishing on a slab. The point in step 1.2 was chosen arbitrarily in the interior, so step 2.1 gives on .
The slice in the first variable. For the fixed of step 1.2, the one-variable map is entire by [F2] and vanishes on the nondegenerate interval by step 3.1. Its zero set therefore has the accumulation point of [F3] inside the domain , and [F1] gives for every .
Conclusion. Since was arbitrary in step 1.2, step 4.1 gives on , which discharges the induction step and completes the induction.
Depends on
Used by
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Sources
- Calder Sheagren, Uncertainty Principles with Fourier Analysis (University of Chicago REU 2017, author PDF) (standard reference, not scraped)