OFFSET
1,5
FORMULA
a(n) = Sum_{k=1..n} mu(n-k)^2 * (ceiling(n/k) - floor(n/k)).
EXAMPLE
a(12) = 5. The numbers less than 12 that do not divide 12 are: {5,7,8,9,10,11} with values of n-k: {7,5,4,3,2,1} (exactly 5 of which are squarefree).
MATHEMATICA
Table[Sum[MoebiusMu[n - k]^2 (Ceiling[n/k] - Floor[n/k]), {k, n}], {n, 100}]
PROG
(PARI) a(n) = sum(k=1, n-1, (n % k) && issquarefree(n-k)); \\ Michel Marcus, Jan 03 2024
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, Jan 02 2024
STATUS
approved