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 A349961 #15 Dec 07 2021 08:28:37
%S A349961 1,3,21,259,4681,111111,3257437,113522235,4581298449,210027483919,
%T A349961 10778947368421,612142982430915,38108188628928601,2580398988131886039,
%U A349961 188802050194014479853,14843696896551724137931,1247923426698972051309601,111713733654631566667971615
%N A349961 a(n) = Sum_{k=0..n} (2*n)^k.
%F A349961 a(n) = ((2*n)^(n+1) - 1)/(2*n - 1).
%t A349961 a[0] = 1; a[n_] := Sum[(2*n)^k, {k, 0, n}]; Array[a, 18, 0] (* _Amiram Eldar_, Dec 07 2021 *)
%o A349961 (PARI) a(n) = sum(k=0, n, (2*n)^k);
%o A349961 (PARI) a(n) = ((2*n)^(n+1)-1)/(2*n-1);
%Y A349961 Cf. A031973, A155956.
%K A349961 nonn,easy
%O A349961 0,2
%A A349961 _Seiichi Manyama_, Dec 07 2021