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

0,4

Comments

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

Links

Table of n, a(n) for n=0..30.

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

Cf. A000110, A056470, A056471, A164904.

Sequence in context: A056470 A056471 A164904 * A245846 A245847 A245848

Adjacent sequences: A188161 A188162 A188163 * A188165 A188166 A188167

Keyword

nonn

Author

Franklin T. Adams-Watters, Mar 23 2011

Status

approved