$\newcommand{\bR}{\mathbb{R}}$ $\newcommand{\ii}{\boldsymbol{i}}$ $\newcommand{\eW}{\mathscr{W}}$ 



While investigating the distribution of critical points of random funtions on tori  I was lead to the following analytical questions.
Suppose that $w :\bR\to [0,\infty)$ is a  Schwartz function. Define

$$  f: \bR\to \bR,\;\; f(t)=w(t^2). $$

We can use the function $f$ to define  for any positive integer $m$ a Schwartz function

$$ F_m:\bR^m\to \bR,\;\;F_m(\vec{x})=f(|\vec{x}|)=w(|\vec{x}|^2). $$

We denote by $\widehat{F}_m(\xi)$ its Fourier transform and by $H_m(w, \vec{\xi})$ the Hessian of $\widehat{F}_m$ at $\vec{\xi}$.

**Problem 1.**   

> Fix a positive integer $m$. Describe the set
>  $\eW_m$  of weights $w$ such
> that
> 
> $$ H_m(w,\xi) < H_m(w, 0),  \;\;
> \forall \vec{\xi}\neq 0. $$


Above, for two symmetric matrices $A$, $B$ the inequality $A< B$ signifies that $B-A$ is positive definite. Hence $w\in\eW_m $ if an only if the origin  is a strict, absolute maximum for the Hessian map  $$ \vec{\xi}\to H_m (w,\vec{\xi}). $$

**Problem 2.** 

> Describe the set
> 
> $$\eW:=\bigcap_{m>0}\eW_m. $$

 Here is some information about $\eW$.  

**A.** For any positive Schwartz function and any $m>0$ we have

$$ H_m(w,\vec{x})< H_m(w,0) $$

if $|\vec{\xi}|$ is sufficiently small, or  sufficiently large. In other words, the origin is always  a local maximum of the Hessian map. Thus the  problem is about what happens in between.

**B.** For any $c>0$  we have $w(s)=e^{-c s/2}\in\eW$. Indeed $f(t)=e^{-ct^2/2}$

$$\widehat{F}_m(\vec{\xi})=const_m e^{-\frac{|\vec{\xi}|^2}{2c}}. $$


**C.** $\eW$ is a convex cone. In particular,  any   linear combination

$$ w(t)=\sum_i A_i e^{-c_it/2},\;\;A_i>0, $$

belongs to $\eW$.  This  implies that if the Schwartz function $w(t)$ is the Laplace transform of a positive finite measure $\mu$ on $[0,\infty)$, then $w(t)\in\eW$.     The classical  *Hausdorff-Bernstein theorem* shows that this happens if and only if $w$ is completely monotone i.e.,

$$(-1)^k w^{(k)}(t)\geq 0,\;;\forall t>0, \;\;\forall k\in\mathbb{Z}_{\geq 0}. $$

Thus $\eW$ contains all the completely monotone Schwartz functions.

Here is a plausible

**Conjecture** (a) The weight $w$ belongs to $\eW$ if and only if $w$ is completely monotone.

(b) $\eW_m \neq \eW$, $\forall m$.