*U*(*f*,*P*) upper sum for definite integral - see Integrals - Theory - Introduction - Riemann integral- uncountable - see Extra - Sets and mappings - Mappings
- uniform continuity - see continuity
- uniform convergence of sequences of functions - see Series - Theory - Series of functions - Sequences of functions and Series - Methods Survey - Series of functions - Sequences of functions
- uniform convergence of series of functions - see Series - Theory - Series of functions - Series of functions and Series - Methods Survey - Series of functions - Series of functions
- union - see Extra - Sets and mappings - Basic set theory
- uniqueness for Fourier series - see Series - Theory - Series of functions - Fourier series
- uniqueness for power series - see Series - Theory - Series of functions - Power series
- uniqueness for Taylor series - see Series - Theory - Series of functions - Taylor series
- upper bound of a function - see boundedness of functions
- upper bound of a set - see Functions - Theory - Real numbers - Topological notions
- upper sum
*U*(*f*,*P*) for definite integral - see Integrals - Theory - Introduction - Riemann integral