The Mean Square of Divisor Function

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