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 A056473 #13 Aug 22 2017 20:53:12
%S A056473 0,0,0,0,0,0,1,1,10,10,65,65,350,350,1701,1701,7770,7770,34105,34105,
%T A056473 145750,145750,611501,611501,2532530,2532530,10391745,10391745,
%U A056473 42355950,42355950,171798901
%N A056473 Number of palindromic structures using exactly four different symbols.
%C A056473 Permuting the symbols will not change the structure.
%D A056473 M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2]
%H A056473 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (1,9,-9,-26,26,24,-24).
%F A056473 stirling2( [(n+1)/2], 4).
%F A056473 G.f.: x^7/((x-1)*(2*x-1)*(2*x+1)*(2*x^2-1)*(3*x^2-1)). [_Colin Barker_, Jul 24 2012]
%F A056473 a(1)=a(2)=a(3)=a(4)=a(5)=a(6)=0, a(7)=1, a(n)=a(n-1)+9*a(n-2)-9*a(n-3)- 26*a(n-4)+ 26*a(n-5)+24*a(n-6)-24*a (n-7). - _Harvey P. Dale_, Mar 08 2013
%t A056473 StirlingS2[Floor[(Range[40]+1)/2],4] (* or *) LinearRecurrence[ {1,9,-9,-26,26,24,-24},{0,0,0,0,0,0,1},40] (* _Harvey P. Dale_, Mar 08 2013 *)
%Y A056473 Cf. A000453, A007581.
%K A056473 nonn,easy
%O A056473 1,9
%A A056473 _Marks R. Nester_