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This is another question on the possible shape of sets $A,B\subset \mathbb{R}^d,d\geq 2,$ where resp. a non-null Schwarz function $f$ and its Fourier transform can vanish.

A nice remark by Christian Remling on Can a function and its Fourier transform both vanish on a convex cone? is that choosing separable functions yields that both functions can vanish on a half-space. The theorem by Shapiro mentioned there says that $A$ and $B$ cannot be resp. a "major convex cone" (strictly larger than a half space) and a non-empty open set.

Hence a remaining question is what can happen if $A$ is a "minor cone", say the quadrant of points with non-negative coordinates. As we saw, $B$ can be a half-space, but not a major cone. Is it possible that $B$ has a "gap in each coordinate"?

Here is a one-sentence question: is it possible to find a non-null Schwartz function $f$ on $\mathbb R^2$ which vanishes on a quarter-space and which Fourier transform $\hat f(x,y)$ vanishes for $x\in O$ or $y\in O'$ where $O$, $O'$ are non-empty open subsets of $\mathbb R$?

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    $\begingroup$ I don't understand your question: Christian Remling's comment which you referred to points out that if $g$ and $\hat h$ both vanish on a half-line, then $f(x,y) := g(x)\,h(y)$ vanishes on a half-plane and $\hat f$ also does. But vanishing on a half-plane is stronger than vanishing on a quarter-plane, so it seems to me that this $f$ exactly answers the question of your last paragraph. $\endgroup$ Commented Nov 20, 2024 at 10:50
  • $\begingroup$ Yes but vanishing on OxO' is stronger (or at least different) than vanishing on a half plane $\endgroup$ Commented Nov 20, 2024 at 10:55
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    $\begingroup$ Now I'm even more confused. Vanishing on $O\times O'$ with $O,O'$ nonempty open sets of $\mathbb{R}$ just means the same as vanishing on a nonempty open set of $\mathbb{R}^2$, since the former are a basis of the latter; and anyway the half-plane we're talking about is of the form $\mathbb{R} \times \mathbb{R}_{\gt 0}$. What more do you want? $\endgroup$ Commented Nov 20, 2024 at 11:07
  • $\begingroup$ Sorry, what I meant by "vanishes on a gap for each coordinate" came out completely wrong, I actually meant that it should vanish on infinite vertical and horizontal gaps, the post edited. $\endgroup$ Commented Nov 20, 2024 at 11:44

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Ok, I think I managed to prove that the answer is no for the discrete version, I put it here for anyone interested (likely generalises to any dimension, didn't look into the continuous version).

Assume $ \psi \subset L^{2}( \mathbb T ^{2})$ has a gap in each coordinate, i.e. vanishes on $T_\varepsilon^c$ where $ T_{\varepsilon }:=( \mathbb T \setminus [-\varepsilon ,\varepsilon ])^{2}$ for some $ \varepsilon >0.$ Then if $ \hat \psi $ vanishes on a quarter space, then $ \psi \equiv 0$.

Let us write the proof by contradiction. Assume wlog that $ \hat \psi $ vanishes on $ (-\mathbb{N}^{*})^{2}$ and that $ \hat \psi (0,0)\neq 0.$

Let $ C_{\varepsilon } \subset \mathbb C $ the unit circle except points of angle in $ [-\varepsilon ,\varepsilon ]$. According to Mergelyan's theorem, there are polynomials $P_{n}( z)\to z^{-1}$ uniformly on $ C_{\varepsilon }$. Putting $ Q_{n}( u)=e^{iu}P_{n}( e^{iu}),$ $ Q_{n}( u)\to 1$ on $ \mathbb T \setminus [-\varepsilon ,\varepsilon ]$, hence $$ Q_{n}(u)Q_{n}(v)\psi (u,v)\to \psi (u,v)$$ in $ T_{\varepsilon } $, hence on $ L^{2} ( \mathbb T ^{2})$ because $ \psi \equiv 0$ on $ T_{\varepsilon }^c$, and $ \hat \psi \star( \hat Q_{n} \otimes \hat Q_{n})\to \hat \psi $ in $ L^{2}( \mathbb{Z} ^{2})$.

Therefore, since $ Q_{n}( u)=\sum_{m>0}a_{m} e^{imu}, \hat Q_{m}=\sum_{m>0}a_{m}\delta _{m}$, \begin{align*} 0\neq \hat \psi ( 0,0)=\lim_n\sum_{k,l\in \mathbb{Z} ^{2}}\underbrace{ \hat \psi ( k,l) }_{=0\text{ if }k<0,l<0}\underbrace{ \hat Q_{n}(- k) \hat Q_{n}( -l)}_{=0\text{ if }k\geqslant0\text{ or }l\geqslant0}=0. \end{align*}

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