OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
FORMULA
EXAMPLE
a(0) = 1: the empty word.
a(1) = 2: a, b for alphabet {a,b}.
a(2) = 4: aa, ab, ba, bb.
a(3) = 5: aaa, aab, aba, baa, bbb.
a(4) = 9: aaaa, aaab, aaba, aabb, abaa, abab, baaa, baab, bbbb.
a(5) = 14: aaaaa, aaaab, aaaba, aaabb, aabaa, aabab, aabba, abaaa, abaab, ababa, baaaa, baaab, baaba, bbbbb.
MAPLE
b:= n-> `if`(n<0, 0, binomial(n, ceil(n/2))):
a:= n-> b(n) +b(n-2) +`if`(n>0, 1, 0):
seq(a(n), n=0..40);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jun 08 2012
STATUS
approved