Egan conjecture

The Egan conjecture in the mathematical area of geometry gives a sufficient and necessary condition for the radii of two spheres and the distance of their centres, so that a simplex exists, which completely contains the larger sphere and completely encloses the smaller sphere. The conjecture generalizes an equality discovered by William Chapple (and later independently by Leonard Euler), which is a special case of Poncelet's closure theorem, as well as the Grace–Danielsson Inequality in one dimension higher. The conjecture is named after the Australian mathematician and science-fiction author Greg Egan.

Basics

For an arbitary triangle ($2$-simplex), the radius $r$ of its inscribed circle, the radius $R$ of its circumcircle and the distance $d$ of their centers are related through Euler's theorem in geometry:

$d^{2}=R(R-2r)$,

which was published by William Chapple in 1746 and by Leonard Euler in 1765.

For two spheres ($2$-spheres) with respective radius $r$ and $R$, fulfilling $r<R$, there exists a (non-regular) tetrahedron ($3$-simplex), which completely contains the larger sphere and completely encloses the smaller sphere, if and only if the distance $d$ of their centers fulfills the Grace–Danielsson Inequality:

$d^{2}\leq (R+r)(R-3r)$.

This result was independently proven by John Hilton Grace in 1917 and G. Danielsson in 1949. A connection with quantum information theory was described by Anthony Milne.

Conjecture

Consider $n$-dimensional euclidean space $\mathbb {R} ^{n}$ for $n\geq 2$. For two $n-1$-spheres with respective radius $r$ and $R$, fulfilling $r<R$, there exists a $n$-simplex, which completely contains the larger sphere and completely encloses the smaller sphere, if and only if the distance $d$ of their centers fulfills:

$d^{2}\leq (R+(n-2)r)(R-nr)$.

The conjecture was proposed by Greg Egan in 2014. For the case $n=1$, where the inequality reduces to $d\leq R-r$, the conjecture is true as well, but trivial. A $0$-sphere is just composed of two points and a $1$-simplex is just a closed interval. The desired $1$-simplex of two given $0$-spheres can simply be chosen as the closed interval between the two points of the larger sphere, which contains the smaller sphere if and only if it contains both of its points with respective distance $|d-r|$ and $d+r$ from the center of the larger sphere, hence if and only if the above inequality is satisfied.

Status

Greg Egan showed that the condition is sufficient under a blog post by John Baez on 16 April 2018. Sergei Drozdov published a paper on ArXiv showing that the condition is also necessary on 16 October 2023.