Four Travelers Problem
Let four Lines in a Plane represent four roads in General Position, and let onetraveler 
be walking along each road at a constant (but not necessarily equal to any other traveler's) speed.Say that two travelers and 
have ``met'' if they were simultaneously at the intersection of their two roads.Then if 
has met all other three travelers (
, 
, and 
) and , in addition to meeting , hasmet and , then and have also met!
References
Bogomolny, A. ``Four Travellers Problem.'' http://www.cut-the-knot.com/gproblems.html.
- Type I Error
- Type II Error
- Ulam Map
- Ulam Number
- Ulam Sequence
- Ulam's Problem
- Ultrametric
- Ultraradical
- Ultraspherical Differential Equation
- Ultraspherical Function
- Ultraspherical Polynomial
- Umbilic Point
- Umbral Calculus
- Umbrella
- Unambiguous
- Unbiased
- Uncia
- Uncorrelated
- Uncountable Set
- Uncountably Infinite Set
- Undecagon
- Undecidable
- Undecillion
- Undetermined Coefficients Method
- Undulating Number