OFFSET
1,1
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.
A118668(a(n)) = 2. - Reinhard Zumkeller, Jul 11 2015
For n > 2, A325907(n) is a term. - Seiichi Manyama, Sep 15 2019
LINKS
Seiichi Manyama and T. D. Noe Table of n, a(n) for n = 1..60 (first 51 terms from Seiichi Manyama)
MATHEMATICA
t = {}; Do[tri = n*(n+1)/2; If[Length[Union[IntegerDigits[tri]]] == 2, AppendTo[t, n]], {n, 10^5}]; t
PROG
(Haskell)
a213518 n = a213518_list !! (n-1)
a213518_list = filter ((== 2) . a118668) [0..]
-- Reinhard Zumkeller, Jul 11 2015
(PARI) for(k=0, 1e8, if(#Set(digits(k*(k+1)/2))==2, print1(k", "))) \\ Seiichi Manyama, Sep 15 2019
CROSSREFS
KEYWORD
nonn,base
AUTHOR
T. D. Noe, Jun 22 2012
EXTENSIONS
a(45)-a(48) from Seiichi Manyama, Sep 15 2019
STATUS
approved