given the sum of divisors of a function
$$ D(x)= \sum_{n=1}^{x}d (n) = x\log x + x(2\gamma-1) + \Delta(x)\ $$
then what is the EXACT formula of $ \Delta (x) $ not the O-notation but the EXACT power series correction to the Dirichlet's divisor problem
where could i find the exact term solution ? is it related (the term) to the Hardy voronoi formula involving the divisor function ? $ d(n) $