Traveling Salesman Problem
A problem in Graph Theory requiring the most efficient (i.e., least total distance) Tour (i.e., closed path)a salesman can take through each of 
cities. No general method of solution is known, and the problem isNP-Hard.
References
Platzman, L. K. and Bartholdi, J. J. ``Spacefilling Curves and the Planar Travelling Salesman Problem.'' J. Assoc. Comput. Mach. 46, 719-737, 1989.
- Overlapping Resonance Method
- Oversampling
- Ovoid
- Paasche's Index
- Packing
- Packing Density
- p-adic Number
- Padovan Sequence
- Padé Approximant
- Padé Conjecture
- Painlevé Property
- Painlevé Transcendents
- Pair
- Paired t-Test
- Pair Sum
- Paley Class
- Paley Construction
- Paley's Theorem
- Palindrome Number
- Palindromic Number
- Palindromic Number Conjecture
- Pancake Cutting
- Pancake Theorem
- Pandiagonal Square
- Pandigital