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 A059108 #37 Nov 22 2018 12:08:04
%S A059108 1,1,0,0,0,0,0,0,0,9,20,33,0,0,0,0,0,0,200343,869006,4247790,0,0,0,0,
%T A059108 0,0
%N A059108 Number of solutions to variant of triples version of Langford (or Langford-Skolem) problem.
%C A059108 How many ways are of arranging the numbers 1,1,1,2,2,2,3,3,3,...,n,n,n so that there are zero numbers between the first and second 1's and zero numbers between the second and third 1's; one number between the first and second 2's and one number between the second and third 2's; ... n-1 numbers between the first and second n's and n-1 numbers between the second and third n's?
%C A059108 a(n)=0 for n mod 9 not in {0,1,2}. - _Gheorghe Coserea_, Aug 23 2017
%H A059108 Gheorghe Coserea, <a href="/A059108/a059108.txt">Solutions for n=10</a>.
%H A059108 Gheorghe Coserea, <a href="/A059108/a059108_1.txt">Solutions for n=11</a>.
%H A059108 J. E. Miller, <a href="http://dialectrix.com/langford.html">Langford's Problem</a>
%e A059108 From _Gheorghe Coserea_, Jul 14 2017: (Start)
%e A059108 For n=9 the a(9)=9 solutions, up to reversal of the order, are:
%e A059108 2 4 2 8 2 4 6 7 9 4 3 8 6 3 7 5 3 9 6 8 5 7 1 1 1 5 9
%e A059108 2 4 2 9 2 4 5 6 7 4 8 5 9 6 3 7 5 3 8 6 3 9 7 1 1 1 8
%e A059108 4 2 5 2 4 2 9 5 4 7 8 3 5 6 3 9 7 3 8 6 1 1 1 7 9 6 8
%e A059108 5 1 1 1 7 5 8 6 9 3 5 7 3 6 8 3 4 9 7 6 4 2 8 2 4 2 9
%e A059108 5 6 1 1 1 5 8 6 9 3 5 7 3 6 8 3 4 9 7 2 4 2 8 2 4 7 9
%e A059108 6 7 9 2 5 2 6 2 7 5 8 9 6 3 5 7 3 4 8 3 9 4 1 1 1 4 8
%e A059108 6 7 9 2 5 2 6 2 7 5 8 9 6 4 5 7 3 4 8 3 9 4 3 1 1 1 8
%e A059108 7 4 2 8 2 4 2 7 9 4 3 8 6 3 7 5 3 9 6 8 5 1 1 1 6 5 9
%e A059108 7 5 3 6 9 3 5 7 3 6 8 5 4 9 7 6 4 2 8 2 4 2 9 1 1 1 8
%e A059108 (End)
%Y A059108 Cf. A014552, A050998, A059106, A059107.
%K A059108 nonn,nice,hard,more
%O A059108 0,10
%A A059108 _N. J. A. Sloane_, Feb 14 2001
%E A059108 _Fausto A. C. Cariboni_ has confirmed the values a(1) to a(20). - _N. J. A. Sloane_, Mar 27 2017
%E A059108 a(21) from _Fausto A. C. Cariboni_, Mar 28 2017