Concept
Bijection
2 essays name this object, across 2 fields. What follows is each of them, and the objects they name alongside it.
A sphere is a plane plus one point
Remove a single point from a sphere and what is left can be flattened out to cover an infinite plane exactly. The construction is one straight line, repeated.
One sequence, counting everything
The number of ways to cut a polygon into triangles is 1, 2, 5, 14, 42. So is the number of ways to bracket a product, the number of binary trees, and the number of paths that never cross a diagonal. They are the same count, and the reason is one picture.
Named alongside it
The objects these essays reach for when they reach for this one.
AstrolabeBinary treesCatalan numbersCircle preservingConformal mapContinuityConvexityLattice pathsPoint at infinityProjectionRecursionRiemann sphere