Concept
Boundary
2 essays name this object, across one field. What follows is each of them, and the objects they name alongside it.
The surface with one side, and what happens when it is cut
A strip of paper, half a twist, and a join. The result has one side and one edge, and cutting it down the middle does not produce two of anything.
Something always stays put
Stir a cup of coffee however violently and let it settle. Some molecule is exactly where it started. Crumple a map and drop it on the region it depicts, and one point lies over the place it names.
Named alongside it
The objects these essays reach for when they reach for this one.
BrouwerContinuityEuler characteristicFixed pointGluing diagramIntermediate value theoremKlein bottleMöbius bandNon orientableNonconstructiveOrientationRetraction