OFFSET
0,5
COMMENTS
a(n) is even for n > 1.
LINKS
Eric Weisstein's World of Mathematics, Langford's Problem
Wikipedia, Dancing Links
Wikipedia, Langford pairing
EXAMPLE
a(1) = 1: 111.
a(4) = 4: 111224234343, 111343432422, 224234343111, 343432422111.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Alois P. Heinz, Dec 06 2018
STATUS
approved