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 A001447 #22 Feb 02 2022 03:58:58
%S A001447 1,3,15,65,325,1575,7875,39125,195625,976875,4884375,24415625,
%T A001447 122078125,610359375,3051796875,15258828125,76294140625,381469921875,
%U A001447 1907349609375,9536744140625,47683720703125
%N A001447 a(n) = (5^n + 5^floor(n/2))/2.
%H A001447 Vincenzo Librandi, <a href="/A001447/b001447.txt">Table of n, a(n) for n = 0..300</a>
%H A001447 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,5,-25).
%F A001447 a(n) = 5*a(n-1) + 5*a(n-2) - 25*a(n-3); a(0)=1, a(1)=3, a(2)=15. - _Harvey P. Dale_, May 05 2011
%F A001447 G.f.: (1 - 2x - 5x^2)/(1 - 5x - 5x^2 + 25x^3). - _Harvey P. Dale_, May 05 2011
%t A001447 Table[(5^n+5^Floor[n/2])/2,{n,0,20}] (* or *) LinearRecurrence[ {5,5,-25}, {1,3,15}, 50](* _Harvey P. Dale_, May 05 2011 *)
%o A001447 (PARI) a(n)=(5^n+5^(n\2))/2 \\ _Charles R Greathouse IV_, Apr 17 2012
%K A001447 nonn,easy
%O A001447 0,2
%A A001447 _N. J. A. Sloane_