OFFSET
1,2
COMMENTS
a(n) is the number of divisors r of 2*n such that r <= n and r and n-r are squarefree. - Robert Israel, Apr 27 2020
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{i=1..n} mu(i)^2 * mu(2*n-i)^2 * (1-ceiling((2*n-i)/i)+floor((2*n-i)/i)), where mu is the Möbius function (A008683).
EXAMPLE
a(6) = 3; 2*6 = 12 has three rectangles with squarefree side lengths, 1 X 11, 2 X 10 and 6 X 6, such that 1 | 11, 2 | 10 and 6 | 6.
MAPLE
f:= proc(n) uses numtheory;
nops(select(r -> r <= n and issqrfree(r) and issqrfree(2*n-r), divisors(2*n), n))
end proc:
map(f, [$1..100]); # Robert Israel, Apr 27 2020
MATHEMATICA
Table[Sum[MoebiusMu[i]^2 MoebiusMu[2 n - i]^2 (1 - Ceiling[(2 n - i)/i] + Floor[(2 n - i)/i]), {i, n}], {n, 100}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, Apr 24 2020
STATUS
approved