OFFSET
0,4
COMMENTS
Permuting the symbols does not change the structure; so e.g. aba and bab are equivalent.
FORMULA
a(2n) = a(2n-1) = B(n), where B(n) = A000110(n) are the Bell numbers.
EXAMPLE
For n=4, the 2 structures are aaaa and abba. For n=5, the 5 structures are aaaaa, aabaa, ababa, abbba, and abcba.
CROSSREFS
KEYWORD
nonn
AUTHOR
Franklin T. Adams-Watters, Mar 23 2011
STATUS
approved