The rhombic dodecahedral conjecture asserts that a regular rhombic dodecahedron has the least surface area among three-dimensional parallelohedra of a fixed inradius. Cesaroni and Novaga (2026) proved the sharp inequality
where
is the surface area and
is the inradius. Equality
holds iff
is similar to a regular rhombic
dodecahedron. This is a fixed-inradius problem,
distinct from minimizing surface area at fixed volume.