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 A117864 #18 Aug 29 2025 08:07:16
%S A117864 4,8,28,48,148,248,748,1248,3748,6248,18748,31248,93748,156248,468748,
%T A117864 781248,2343748,3906248,11718748,19531248,58593748,97656248,292968748,
%U A117864 488281248,1464843748,2441406248,7324218748,12207031248,36621093748,61035156248
%N A117864 Number of palindromes (in base 5) below 5^n.
%H A117864 Harvey P. Dale, <a href="/A117864/b117864.txt">Table of n, a(n) for n = 1..1000</a>
%H A117864 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,5,-5).
%F A117864 a(n) = 6*5^((n-1)/2)-2 (n odd), 2*5^(n/2)-2 (n even).
%F A117864 a(1)=4, a(2)=8, a(3)=28, a(n)=a(n-1)+5*a(n-2)-5*a(n-3). - _Harvey P. Dale_, May 30 2012
%F A117864 G.f.: 4*x*(x+1) / ((x-1)*(5*x^2-1)). - _Colin Barker_, Apr 26 2015
%F A117864 a(n) = 4*A238366(n-1). - _Michel Marcus_, Apr 26 2015
%F A117864 E.g.f.: 2*cosh(sqrt(5)*x) - 2*cosh(x) - 2*sinh(x) + 6*sinh(sqrt(5)*x)/sqrt(5). - _Stefano Spezia_, Aug 29 2025
%t A117864 pal5s[n_]:=If[OddQ[n],6 5^((n-1)/2)-2,2 5^(n/2)-2]; Array[pal5s,40] (* or *) LinearRecurrence[{1,5,-5},{4,8,28},40] (* _Harvey P. Dale_, May 30 2012 *)
%o A117864 (PARI) Vec(4*x*(x+1)/((x-1)*(5*x^2-1)) + O(x^100)) \\ _Colin Barker_, Apr 26 2015
%Y A117864 Cf. A050250, A238366.
%K A117864 nonn,base,easy,changed
%O A117864 1,1
%A A117864 _Martin Renner_, May 02 2006
%E A117864 More terms from _Harvey P. Dale_, May 30 2012