An approximate counterexample to the Barker--Larman problem in dimension $4$

Abstract: The Barker--Larman problem asks if a convex body $K \subseteq \mathbb R<sup>n$ containing the Euclidean ball $\mathbb B_n$, such that all the sections of $K$ by hyperplanes tangent to $\mathbb B_n$ have constant $(n-1)$-dimensional volume, must necessarily be a Euclidean ball. In this paper we show a result pointing to a negative answer in dimension $4$. Taking $λ<em>0 = 4 \sqrt{3}π$ and any $N \in \mathbb N$, we obtain the existence of a family of convex bodies $K</em>{λ,N}$ with $λ\in (λ<em>0-r_N, λ_0 + r_N)$, such that the sections of $K</em>{λ,N}$ by hyperplanes tangent to the Euclidean ball, have area within $c |λ- λ<em>0|<sup>{\frac{N+1}2}$ of $λ$, while the difference between outradius and inradius of $K</em>{λ,N}$ is larger than $C |λ- λ<em>0|$. The bodies $K</em>{λ,N}$ are constructed via radial functions as [ρ{K{λ,N}}(t) = \cos\left( \sum_{n=0}^N \frac{(λ-λ_0)^n}{n!} \varphi_n(t) \right)^{-1},] where $t \in [0,2π), λ\in \mathbb R$ and $\varphi_n$ are trigonometric polynomials that can be computed explicitly. The convergence of the inner power series when $N \to \infty$ (which is left open) would imply a negative answer to the Barker--Larman problem in dimension $4$. As an example we obtain a convex body whose outradius and inradius differ by more than $0.176$, and the area of the sections oscillate by less than $3 \times 10<sup>{-7}$.