EN
Let d(n) be the divisor function. In 1916, S. Ramanujan stated without proof that
$∑_{n≤x} d²(n) = xP(log x) + E(x)$,
where P(y) is a cubic polynomial in y and
$E(x) = O(x^{3/5 + ε})$,
with ε being a sufficiently small positive constant. He also stated that, assuming the Riemann Hypothesis (RH),
$E(x)=O(x^{1/2 + ε})$.
In 1922, B. M. Wilson proved the above result unconditionally. The direct application of the RH would produce
$E(x) = O(x^{1/2}(log x)⁵loglog x)$.
In 2003, K. Ramachandra and A. Sankaranarayanan proved the above result without any assumption.
In this paper, we prove
$E(x) = O(x^{1/2}(log x)⁵)$.