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 A054889 #15 Apr 19 2023 02:16:11
%S A054889 1,5,20,70,245,860,3015,10570,37060,129935,455560,1597225,5599980,
%T A054889 19633910,68837825,241350100,846189875,2966799290,10401800220,
%U A054889 36469419475,127864266640,448300820765,1571773187140,5510743762630
%N A054889 Layer counting sequence for hyperbolic tessellation by regular pentagons of angle 2*Pi/5.
%C A054889 The layer sequence is the sequence of the cardinalities of the layers accumulating around a (finite-sided) polygon of the tessellation under successive side-reflections; see the illustration accompanying A054888.
%H A054889 Georg Fischer, <a href="/A054889/b054889.txt">Table of n, a(n) for n = 1..500</a>
%H A054889 <a href="/index/Con#coordseqs">Index entries for Coordination Sequences</a> [A layer sequence is a kind of coordination sequence. - _N. J. A. Sloane_, Nov 20 2022]
%H A054889 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,1,3,-1).
%F A054889 G.f.: x*(1+2*x+4*x^2+2*x^3+x^4)/(1-3*x-x^2-3*x^3+x^4).
%t A054889 LinearRecurrence[{3,1,3,-1},{1,5,20,70,245},40] (* _Georg Fischer_, Apr 13 2020 *)
%o A054889 (Magma) R<x>:=PowerSeriesRing(Integers(), 40); Coefficients(R!( x*(1+2*x+4*x^2+2*x^3+x^4)/(1-3*x-x^2-3*x^3+x^4) )); // _G. C. Greubel_, Feb 08 2023
%o A054889 (Sage)
%o A054889 def A054889_list(prec):
%o A054889 P.<x> = PowerSeriesRing(ZZ, prec)
%o A054889 return P( x*(1+2*x+4*x^2+2*x^3+x^4)/(1-3*x-x^2-3*x^3+x^4) ).list()
%o A054889 a=A054889_list(40); a[1:] # _G. C. Greubel_, Feb 08 2023
%Y A054889 Cf. A054886, A054887, A054888, A054890.
%K A054889 nonn,easy
%O A054889 1,2
%A A054889 Paolo Dominici (pl.dm(AT)libero.it), May 23 2000
%E A054889 a(21) inserted by _Georg Fischer_, Apr 13 2020