OFFSET
1,2
COMMENTS
EXAMPLE
a(2) = 2 since the arrangements 1,3,2,4 and 1,3,4,2 both satisfy the condition.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Bennett Gardiner, Jun 16 2011
EXTENSIONS
a(7) corrected, a(8)-a(13) added by Max Alekseyev, Aug 19 2013
STATUS
approved