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A188164
Number of palindromic structures of length n.
3
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.
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
STATUS
approved