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 A004157 #26 Mar 31 2024 15:31:07
%S A004157 0,1,3,6,1,6,3,10,9,9,10,12,15,10,6,3,10,9,9,10,3,6,10,15,3,10,9,18,
%T A004157 10,12,15,19,15,12,19,9,18,10,12,15,10,15,12,19,18,9,10,12,15,10,15,
%U A004157 12,19,9,18,10,21,15,10,15,12,19,18,9,10,12,6,19,15,12,19,18,18,10,21,15,19,6,12,10
%N A004157 Sum of digits of n-th triangular number.
%H A004157 Robert Israel, <a href="/A004157/b004157.txt">Table of n, a(n) for n = 0..10000</a>
%F A004157 a(n) = A007953(A000217(n)). - _Robert Israel_, Jun 18 2015
%p A004157 seq(convert(convert(n*(n+1)/2,base,10),`+`), n=0..100); # _Robert Israel_, Jun 18 2015
%t A004157 Table[Plus@@IntegerDigits@(n (n + 1) / 2), {n, 0, 90}] (* _Vincenzo Librandi_, Jun 18 2015 *)
%t A004157 Total[IntegerDigits[#]]&/@Accumulate[Range[0,100]] (* _Harvey P. Dale_, Mar 31 2024 *)
%o A004157 (Magma) [&+Intseq(n*(n+1) div 2): n in [0..80] ]; // _Vincenzo Librandi_, Jun 18 2015
%o A004157 (PARI) a(n) = sumdigits(n*(n+1)/2); \\ _Michel Marcus_, Jun 18 2015
%Y A004157 Cf. A000217, A007953, A062688, A145389.
%K A004157 nonn,base
%O A004157 0,3
%A A004157 _N. J. A. Sloane_