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# Index to OEIS: Section U

# Index to OEIS: Section U

- This is a section of the Index to the
**OEIS®**. - For further information see the main
**Index to OEIS**page. - Please read Index: Instructions For Updating Index to OEIS before making changes to this page.
- If you did not find what you were looking for in this Index, you can always search the database for a particular word or phrase.
- Full list of sections:

[ Aa | Ab | Al | Am | Ap | Ar | Ba | Be | Bi | Bl | Bo | Br | Ca | Ce | Ch | Cl | Coa | Coi | Com | Con | Cor | Cu | Cy | Da | De | Di | Do | Ea | Ed | El | Eu | Fa | Fe | Fi | Fo | Fu | Ga | Ge | Go | Gra | Gre | Ha | He | Ho | Ia | In | J | K | La | Lc | Li | Lo | Lu | M | Mag | Map | Mat | Me | Mo | Mu | N | Na | Ne | Ni | No | Nu | O | Pac | Par | Pas | Pea | Per | Ph | Poi | Pol | Pos | Pow | Pra | Pri | Pro | Ps | Qua | Que | Ra | Rea | Rel | Res | Ro | Ru | Sa | Se | Si | Sk | So | Sp | Sq | St | Su | Sw | Ta | Te | Th | To | Tra | Tri | Tu | U | V | Wa | We | Wi | X | Y | Z | 1 | 2 | 3 | 4 ]

U-coordinates for arrays: see T-coordinates for arrays

ugly numbers: A051037

** Ulam numbers: sequences related to : **

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 function uphi: see unitary totient function

unitary totient function uphi: A047994*, A049865, A003271

universal quadratic forms: A030050, A030051, A116582, A154363

unpredictable sequence: A007061

** unsolved problems in number theory (selected): : **

- 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*

- This is a section of the Index to the
**OEIS®**. - For further information see the main
**Index to OEIS**page. - Please read Index: Instructions For Updating Index to OEIS before making changes to this page.
- If you did not find what you were looking for in this Index, you can always search the database for a particular word or phrase.
- Full list of sections:

[ Aa | Ab | Al | Am | Ap | Ar | Ba | Be | Bi | Bl | Bo | Br | Ca | Ce | Ch | Cl | Coa | Coi | Com | Con | Cor | Cu | Cy | Da | De | Di | Do | Ea | Ed | El | Eu | Fa | Fe | Fi | Fo | Fu | Ga | Ge | Go | Gra | Gre | Ha | He | Ho | Ia | In | J | K | La | Lc | Li | Lo | Lu | M | Mag | Map | Mat | Me | Mo | Mu | N | Na | Ne | Ni | No | Nu | O | Pac | Par | Pas | Pea | Per | Ph | Poi | Pol | Pos | Pow | Pra | Pri | Pro | Ps | Qua | Que | Ra | Rea | Rel | Res | Ro | Ru | Sa | Se | Si | Sk | So | Sp | Sq | St | Su | Sw | Ta | Te | Th | To | Tra | Tri | Tu | U | V | Wa | We | Wi | X | Y | Z | 1 | 2 | 3 | 4 ]