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 A178186 #12 Mar 25 2023 12:38:18
%S A178186 9,252,19935,4802904,3491587305,7629089072292,50039174188071999,
%T A178186 984820941357799304880,58150721823981417489695049,
%U A178186 10301109611599361435391036962892,5474411390529830981438591324606714655
%N A178186 Sum 3^((k^2+3k)/2) from k=1 to n.
%C A178186 Series of the kind m^((k^2+3k)/2) from k=1 to n was studied by Bernoulli and Euler.
%H A178186 Vincenzo Librandi, <a href="/A178186/b178186.txt">Table of n, a(n) for n = 1..60</a>
%t A178186 aa = {}; m = 3; sum = 0; Do[sum = sum + m^((n + 3) n/2); AppendTo[aa, sum], {n, 1, 20}]; aa (*Artur Jasinski*)
%t A178186 Table[Sum[3^((k^2+3k)/2),{k,n}],{n,20}] (* _Harvey P. Dale_, Jul 10 2020 *)
%t A178186 Accumulate[Table[3^((k^2+3k)/2),{k,15}]] (* _Harvey P. Dale_, Mar 25 2023 *)
%o A178186 (PARI) a(n) = sum(k=1, n, 3^((k^2+3*k)/2)); \\ _Michel Marcus_, Sep 09 2013
%Y A178186 Cf. A178184-A178193.
%K A178186 nonn
%O A178186 1,1
%A A178186 _Artur Jasinski_, May 21 2010