OFFSET
1,3
COMMENTS
The number of geometric subsequences of [1,...,n] with integral successive-term ratio r and length k is floor(n/r^(k-1))(n > 0, r > 1, k > 0).
FORMULA
a(n) = sum {r > 1, j > 0} floor(n/r^j)
EXAMPLE
a(2): [1,2]; a(3): [1,2],[1,3]; a(4): [1,2],[1,3],[1,4],[2,4],[1,2,4]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Robert E. Sawyer (rs.1(AT)mindspring.com)
STATUS
approved