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 A117857 #14 Apr 18 2019 16:45:58
%S A117857 4,4,20,20,100,100,500,500,2500,2500,12500,12500,62500,62500,312500,
%T A117857 312500,1562500,1562500,7812500,7812500,39062500,39062500,195312500,
%U A117857 195312500,976562500,976562500,4882812500,4882812500,24414062500,24414062500,122070312500
%N A117857 Number of palindromes of length n (in base 5).
%H A117857 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,5).
%F A117857 a(n) = 4*5^floor((n-1)/2).
%F A117857 a(n) = 5*a(n-2). G.f.: -4*x*(x+1)/(5*x^2-1). [_Colin Barker_, Feb 15 2013]
%p A117857 A117857:=n->4*5^floor((n-1)/2): seq(A117857(n), n=1..40); # _Wesley Ivan Hurt_, Apr 18 2017
%t A117857 LinearRecurrence[{0,5},{4,4},40] (* or *) With[{c=NestList[ 5#&,4,20]},Riffle[ c,c]] (* _Harvey P. Dale_, Apr 18 2019 *)
%Y A117857 Cf. A050683.
%K A117857 nonn,base,easy
%O A117857 1,1
%A A117857 _Martin Renner_, May 02 2006
%E A117857 More terms from _Colin Barker_, Feb 15 2013