A320392 Number of permutations of 3 indistinguishable copies of 1,...,n such that there are exactly j numbers between the first and the third copy of j and floor(j/2) numbers between the first and the second or between the second and the third copy of j.
1, 1, 0, 0, 4, 4, 10, 24, 252, 410, 1998, 7798, 65188, 280582, 2281108, 10585748, 110903088
Offset: 0
Examples
a(1) = 1: 111. a(4) = 4: 111224234343, 111343432422, 224234343111, 343432422111.
Links
- Eric Weisstein's World of Mathematics, Langford's Problem
- Wikipedia, Dancing Links
- Wikipedia, Langford pairing
Comments