Problem

Source: Replacement IMO 1980 in Mersch - P5 - BE5

Tags: geometry, 3D geometry, sphere, function, combinatorics proposed, combinatorics



In the Euclidean three-dimensional space, we call folding of a sphere $S$ every partition of $S \setminus \{x,y\}$ into disjoint circles, where $x$ and $y$ are two points of $S$. A folding of $S$ is called linear if the circles of the folding are obtained by the intersection of $S$ with a family of parallel planes or with a family of planes containing a straight line $D$ exterior to $S$. Is every folding of a sphere $S$ linear?