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# Index to OEIS: Section U

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# 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
]