OFFSET
1,3
COMMENTS
Permuting the alphabet will not change a word structure. Thus aabc and bbca have the same structure.
REFERENCES
M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2]
FORMULA
a(n) = Sum_{d|n} mu(d)*A007581(n/d-1).
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved