OFFSET
0,2
EXAMPLE
a(0) = 1: the empty word.
a(1) = 720: abcdef, abcdfe, abcedf, ..., fedbca, fedcab, fedcba.
a(2) = 10395: aabbccddeeff, aabbccddefef, aabbccddfeef, ..., fedcaabbcdef, fedcababcdef, fedcbaabcdef.
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jun 23 2012
STATUS
approved