Fermat 4n+1 Theorem
Every Prime of the form 
is a sum of two Square Numbers in one unique way (up to the order ofFermat 
, but the first published proof was byEuler.
References
Hardy, G. H. and Wright, E. M. ``Some Notation.'' Th. 251 in An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Clarendon Press, 1979.
- Order (Ordinary Differential Equation)
- Order (Permutation)
- Order (Polynomial)
- Order Statistic
- Order (Vertex)
- Ordinal Number
- Ordinary Differential Equation
- Ordinary Differential Equation--First-Order
- Ordinary Differential Equation--First-Order Exact
- Ordinary Differential Equation Second-Order
- Ordinary Differential Equation--System with Constant Coefficients
- Ordinary Double Point
- Ordinary Line
- Ordinary Point
- Ordinate
- Ore Number
- Ore's Conjecture
- Ore's Theorem
- Orientable Surface
- Orientation (Plane Curve)
- Orientation-Preserving
- Orientation-Reversing
- Orientation (Vectors)
- Origami
- Origin