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 A247840 #27 Jan 14 2025 14:58:33
%S A247840 0,36,252,1548,9324,55980,335916,2015532,12093228,72559404,435356460,
%T A247840 2612138796,15672832812,94036996908,564221981484,3385331888940,
%U A247840 20311991333676,121871948002092,731231688012588,4387390128075564,26324340768453420
%N A247840 a(n) = Sum_{k=2..n} 6^k.
%H A247840 Vincenzo Librandi, <a href="/A247840/b247840.txt">Table of n, a(n) for n = 1..200</a>
%H A247840 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (7,-6).
%F A247840 G.f.: 36*x^2/((1-x)*(1-6*x)).
%F A247840 a(n) = a(n-1) + 6^n = (6^(n+1) - 36)/5 = 7*a(n-1) - 6*a(n-2).
%F A247840 a(n) = A105281(n) - 6. - _Michel Marcus_, Sep 25 2014
%F A247840 a(n) = 36 * A003464(n-1). - _Alois P. Heinz_, Jan 14 2025
%t A247840 RecurrenceTable[{a[1] == 0, a[n] == a[n-1] + 6^n}, a, {n, 30}] (* or *) CoefficientList[Series[36 x / ((1 - x) (1 - 6 x)), {x, 0, 30}], x]
%t A247840 Join[{0},Accumulate[6^Range[2,30]]] (* or *) LinearRecurrence[{7,-6},{0,36},30] (* _Harvey P. Dale_, Jun 11 2016 *)
%o A247840 (Magma) [0] cat [&+[6^k: k in [2..n]]: n in [2..30]];
%o A247840 (Magma) [(6^(n+1)-36)/5: n in [1..30]];
%o A247840 (PARI) a(n) = sum(k=2, n, 6^k); \\ _Michel Marcus_, Sep 25 2014
%Y A247840 Cf. similar sequences listed in A247817.
%Y A247840 Cf. A000400, A003464, A105281.
%K A247840 nonn,easy
%O A247840 1,2
%A A247840 _Vincenzo Librandi_, Sep 25 2014