U-coordinates for arrays: see T-coordinates for arrays
ugly numbers: A051037
Ulam numbers: sequences related to (start):
Ulam numbers: A002858 * A078425
Ulam-type sequences: A002858 *, A001857 , A003664 , A007300 , A003668 , A002859 , A003669 , A003666 , A006844 , A003662 , A003670 , A003667 , A003663 , A117140
unary representations: A000042
unary-binary tree, definition: A029766
undecagon is spelled 11-gon in the OEIS
undulating numbers: A033619 *
undulating squares: A016073
unexplained differences between partition generating functions: A007326 , A007327 , A007328 , A007329 , A007330
unhappy numbers: A031177
unidecagon is spelled 11-gon in the OEIS
Unions and sums:: A003430
unique factorization domains: A003172 *
unit fractions: see Egyptian fractions
unit interval graphs: see interval graphs
unitary amicable numbers: A002952 *, A002953 *
unitary divisors of n, number of: A034444 *
unitary divisors of n, sum of: A034448 *
unitary divisors: see also (1) A002827 A033854 A033857 A033858 A033859 A034460 A034676 A034677 A034678 A034679 A034680 A034681
unitary divisors: see also (2) A034682 A034684 A037176 A046971
unitary perfect numbers: A002827 *
unitary phi (or unitary totient) function uphi: A047994 *
unpredictable sequence: A007061
unsolved problems in number theory (selected): (start):
unsolved problems in number theory (selected): A000069 , A001220 , A001969 , A002496 , A036262 , A070087 , A070089 , A070176 , A07019, A144914
unsolved problems in number theory: see also Goldbach conjecture
unsolved problems in number theory: see also Riemann hypothesis
unsolved problems in number theory: see also: sequences that need extending
untouchable numbers: A005114 *
up-down permutations: A000111 *
urinals: see pay-phones
urns: A003125 , A003126 , A003127 , A063169 , A063170
usigma(n): A034448 *