Orthogonal Basis
A Basis of vectors 
which satisfy
where
, 
are constants (not necessarily equal to 1) and 
is the Kronecker Delta.See also Basis, Orthonormal Basis- Double Bubble
- Double Bubble Conjecture
- Double Contraction Relation
- Double Cusp
- Double Exponential Distribution
- Double Exponential Integration
- Double Factorial
- Double Folium
- Double-Free Set
- Double Gamma Function
- Double Point
- Double Sixes
- Double Sum
- Doublet Function
- Doubly Even Number
- Doubly Magic Square
- Dougall-Ramanujan Identity
- Dougall's Theorem
- Doughnut
- Douglas-Neumann Theorem
- Dovetailing Problem
- Dowker Notation
- Down Arrow Notation
- Dozen
- Dragon Curve