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 A062099 #19 Dec 05 2024 17:58:31
%S A062099 0,1,3,6,10,15,21,28,55,78,91,105,120,136,190,210,231,253,276,300,325,
%T A062099 406,465,528,703,780,820,861,1081,1176,1225,1275,1540,1596,1653,1711,
%U A062099 1770,2080,2211,2346,2701,2775,2850,3003,3160,3403,3486,3570,3741,3828
%N A062099 Triangular numbers whose sum of digits is a triangular number.
%H A062099 Harry J. Smith, <a href="/A062099/b062099.txt">Table of n, a(n) for n = 1..1000</a>
%e A062099 a(8) = 28 is a triangular number and the sum of digits 10 is also a triangular number.
%t A062099 With[{trnos=Accumulate[Range[0,200]]},Select[trnos,MemberQ[trnos, Total[ IntegerDigits[ #]]]&]] (* _Harvey P. Dale_, Feb 26 2013 *)
%o A062099 (PARI) { for(m=0, 100, my(k=binomial(m+1,2)); if(ispolygonal(sumdigits(k),3), print1(k, ", "))) } \\ _Harry J. Smith_, Aug 01 2009
%o A062099 (Magma) [ t: n in [0..90] | IsSquare(8*s+1) where s is &+Intseq(t) where t is n*(n+1) div 2 ]; // _Bruno Berselli_, May 27 2011
%Y A062099 Intersection of A187744 and A000217.
%K A062099 nonn,base,easy
%O A062099 1,3
%A A062099 _Amarnath Murthy_, Jun 16 2001
%E A062099 More terms from _Erich Friedman_, Jun 20 2001