login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A213517 Numbers n such that the triangular number n*(n+1)/2 has only 1 or 2 different digits in base 10. 4
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 18, 24, 34, 36, 44, 58, 66, 77, 100, 101, 105, 109, 114, 132, 141, 363, 666, 714, 816, 1000, 1095, 1287, 1332, 1541, 3363, 6666, 10000, 10114, 13332, 66666, 100000, 133332, 666666, 1000000, 1333332, 6666666, 10000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

The list of triangular numbers containing only one digit (A045914) is finite. This list is infinite because numbers like 133332, 666666, and 1000000 occur an infinite number of times. Are there more terms not of this form? See A213516 for corresponding triangular numbers.

A118668(a(n)) <= 2. - Reinhard Zumkeller, Jul 11 2015

LINKS

Table of n, a(n) for n=1..51.

MATHEMATICA

t = {}; Do[tri = n*(n+1)/2; If[Length[Union[IntegerDigits[tri]]] <= 2, AppendTo[t, n]], {n, 0, 10^5}]; t

PROG

(Haskell)

a213517 n = a213517_list !! (n-1)

a213517_list = filter ((<= 2) . a118668) [0..]

-- Reinhard Zumkeller, Jul 11 2015

CROSSREFS

Cf. A045914, A213516.

Cf. A118668.

Sequence in context: A154125 A106039 A151767 * A173899 A108897 A023767

Adjacent sequences:  A213514 A213515 A213516 * A213518 A213519 A213520

KEYWORD

nonn,base

AUTHOR

T. D. Noe, Jun 21 2012

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 21 17:17 EDT 2019. Contains 323444 sequences. (Running on oeis4.)