Segre's Theorem
For any Real Number 
, an Irrational number 
can be approximated byinfinitely many Rational fractions 
in such a way that
If
, this becomes Hurwitz's Irrational Number Theorem.See also Hurwitz's Irrational Number Theorem- Uniform Polyhedron
- Uniform Variate
- Unimodal Distribution
- Unimodal Sequence
- Unimodular Group
- Unimodular Matrix
- Unimodular Transformation
- Union
- Uniplanar Double Point
- Unipotent
- Unique
- Unique Factorization Theorem
- Unit
- Unital
- Unitary Aliquot Sequence
- Unitary Amicable Pair
- Unitary Divisor
- Unitary Group
- Unitary Matrix
- Unitary Multiperfect Number
- Unitary Multiplicative Character
- Unitary Perfect Number
- Unitary Sociable Numbers
- Unitary Transformation
- Unitary Unimodular Group