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A234848
Triangular numbers with digits in nondecreasing order.
2
0, 1, 3, 6, 15, 28, 36, 45, 55, 66, 78, 136, 378, 666, 1128, 1225, 1378, 2278, 2346, 2556, 5778, 12246, 13366, 22366, 22578, 35778, 111156, 222778, 223446, 333336, 1113778, 1222266, 1457778, 2235555, 3557778, 22227778, 22234446, 111116778, 156777778, 222446778
OFFSET
1,3
COMMENTS
Beyond 222446778, all terms are k(k+1)/2 for k = 2s7, 6s7, or 6s8, where s stands for any number of 6's. - T. D. Noe, Jan 01 2014
LINKS
T. D. Noe, Table of n, a(n) for n = 1..328 (terms less than 10^200)
MATHEMATICA
inOrder[nums_] := Min[Differences[nums]] >= 0; t = {}; Do[tri = n*(n+1)/2; If[inOrder[IntegerDigits[tri]], AppendTo[t, tri]], {n, 0, 10^5}]; t (* T. D. Noe, Dec 31 2013 *)
Select[Accumulate[Range[0, 22000]], Min[Differences[IntegerDigits[#]]]>=0&] (* Harvey P. Dale, Apr 06 2023 *)
PROG
(Python)
from itertools import chain, count, islice, combinations_with_replacement
from sympy import integer_nthroot
def A234848_gen(): # generator of terms
return chain((0, ), (n for n in (int(''.join(i)) for l in count(1) for i in combinations_with_replacement('123456789', l)) if integer_nthroot(8*n+1, 2)[1]))
A234848_list = list(islice(A234848_gen(), 50)) # Chai Wah Wu, May 22 2022
CROSSREFS
Sequence in context: A000740 A056278 A161625 * A300761 A069712 A076971
KEYWORD
base,nonn
AUTHOR
Zak Seidov, Dec 31 2013
STATUS
approved