Mertens Theorem
where
is the Euler-Mascheroni Constant and 
.
References
Hardy, G. H. and Wright, E. M. An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Oxford University Press, p. 351, 1979. Riesel, H. Prime Numbers and Computer Methods for Factorization, 2nd ed. Boston, MA: Birkhäuser, pp. 66-67, 1994.
- Uncountable Set
- Uncountably Infinite Set
- Undecagon
- Undecidable
- Undecillion
- Undetermined Coefficients Method
- Undulating Number
- Unduloid
- Unexpected Hanging Paradox
- Unfinished Game
- Unhappy Number
- Unicursal Circuit
- Uniform Apodization Function
- Uniform Boundedness Principle
- Uniform Convergence
- Uniform Distribution
- Uniform Polyhedron
- Uniform Variate
- Unimodal Distribution
- Unimodal Sequence
- Unimodular Group
- Unimodular Matrix
- Unimodular Transformation
- Union
- Uniplanar Double Point