Riemannian Manifold
A Manifold possessing a Metric Tensor. For a complete Riemannian manifold, the Metric 
isdefined as the length of the shortest curve (Geodesic) between 
and 
.
- Reciprocal Polyhedron
- Reciprocating Sphere
- Reciprocation
- Reciprocity Theorem
- Rectangle
- Rectangle Function
- Rectangle Squaring
- Rectangular Coordinates
- Rectangular Distribution
- Rectangular Hyperbola
- Rectangular Parallelepiped
- Rectangular Projection
- Rectifiable Current
- Rectifiable Set
- Rectification
- Rectifying Latitude
- Rectifying Plane
- Recurrence Relation
- Recurrence Sequence
- Recurring Digital Invariant
- Recursion
- Recursive Function
- Recursive Monotone Stable Quadrature
- Red-Black Tree
- Red Net