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 A056457 #20 Sep 28 2018 10:01:45
%S A056457 0,0,0,0,0,0,0,0,0,0,720,720,15120,15120,191520,191520,1905120,
%T A056457 1905120,16435440,16435440,129230640,129230640,953029440,953029440,
%U A056457 6711344640,6711344640,45674188560
%N A056457 Palindromes using exactly six different symbols.
%D A056457 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 A056457 <a href="/index/Rec#order_11">Index entries for linear recurrences with constant coefficients</a>, signature (1,20,-20,-155,155,580,-580,-1044,1044,720,-720).
%F A056457 a(n) = 6! * Stirling2( [(n+1)/2], 6).
%F A056457 G.f.: 720*x^11/((x-1)*(2*x-1)*(2*x+1)*(2*x^2-1)*(3*x^2-1)*(5*x^2-1)*(6*x^2-1)). - _Colin Barker_, Sep 03 2012
%F A056457 G.f.: k!(x^(2k-1)+x^(2k))/Product_{i=1..k}(1-ix^2), where k=6 is the number of symbols. - _Robert A. Russell_, Sep 25 2018
%t A056457 k=6; Table[k! StirlingS2[Ceiling[n/2],k],{n,1,30}]
%o A056457 (PARI) a(n) = 6!*stirling((n+1)\2, 6, 2); \\ _Altug Alkan_, Sep 25 2018
%Y A056457 Cf. A056452, A000920.
%K A056457 nonn,easy
%O A056457 1,11
%A A056457 _Marks R. Nester_