OFFSET
0,2
EXAMPLE
a(0) = 1: the empty word.
a(1) = 5040: abcdefg, abcdegf, abcdfeg, ..., gfedbca, gfedcab, gfedcba, (all permutations of 7 letters).
a(2) = 135135: aabbccddeeffgg, aabbccddeefgfg, aabbccddeegffg, ..., gfedcaabbcdefg, gfedcababcdefg, gfedcbaabcdefg.
CROSSREFS
KEYWORD
nonn,changed
AUTHOR
Alois P. Heinz, Jun 23 2012
STATUS
approved