Pépin's Theorem
The Fermat Number 
is Prime Iff
See also Fermat Number, Pépin's Test, Selfridge-Hurwitz Residue
- Unknot
- Unknotting Number
- Unless
- Unlesss
- Unmixed
- Unpoke Move
- Unsafe
- Unsolved Problem
- Unstable Improper Node
- Unstable Node
- Unstable Spiral Point
- Unstable Star
- Untouchable Number
- U-Number
- Upper Bound
- Upper Integral
- Upper Limit
- Upper Sum
- Upper-Trimmed Subsequence
- Upward Drawing
- Urchin
- Utility Graph
- Utility Problem
- Valence
- Valency