A188164 Number of palindromic structures of length n.

1, 1, 1, 2, 2, 5, 5, 15, 15, 52, 52, 203, 203, 877, 877, 4140, 4140, 21147, 21147, 115975, 115975, 678570, 678570, 4213597, 4213597, 27644437, 27644437, 190899322, 190899322, 1382958545, 1382958545

Offset: 0

Author

Franklin T. Adams-Watters, Mar 23 2011

Keywords

• nonn

Comments

Permuting the symbols does not change the structure; so e.g. aba and bab are equivalent.

Examples

			For n=4, the 2 structures are aaaa and abba. For n=5, the 5 structures are aaaaa, aabaa, ababa, abbba, and abcba.
		

Crossrefs

Cf. A000110, A056470, A056471, A164904.

Formula

a(2n) = a(2n-1) = B(n), where B(n) = A000110(n) are the Bell numbers.