This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A117511 #16 Aug 30 2025 04:15:25
%S A117511 36,153,2556,3240,4851,5778,9045,11628,13041,14535,17766,19503,33930,
%T A117511 41328,46665,49455,52326,71253,74691,81810,85491,93096,109278,122265,
%U A117511 131328,140715,145530,160461,170820,181503,186966,192510,203841,252405,258840,265356
%N A117511 Triangular numbers for which the sum of the digits equals the sum of the digits of the next triangular number.
%C A117511 Each term is divisible by 9.
%H A117511 Harvey P. Dale, <a href="/A117511/b117511.txt">Table of n, a(n) for n = 1..1000</a>
%F A117511 s(a(n)) = s(a(n+1)), where s(n) is the sum of the digits of n.
%e A117511 153 is in the sequence because (1) 153 is triangular number a(18), triangular number a(19)=171 and (2) 1+5+3=1+7+1.
%t A117511 Transpose[With[{c=Partition[Accumulate[Range[2000]],2,1]}, Select[c, Total[IntegerDigits[First[#]]]==Total[IntegerDigits[Last[#]]]&]]] [[1]] (* _Harvey P. Dale_, Oct 18 2011 *)
%t A117511 (#(#+1))/2&/@(SequencePosition[Total[IntegerDigits[#]]&/@Accumulate[ Range[ 1000]],{x_,x_}][[All,1]]) (* _Harvey P. Dale_, Mar 02 2022 *)
%Y A117511 Cf. A000217, A004157.
%K A117511 base,nonn,changed
%O A117511 1,1
%A A117511 Luc Stevens (lms022(AT)yahoo.com), Apr 26 2006
%E A117511 Corrected by _Harvey P. Dale_, Oct 18 2011