Divisor summatory function
In number theory, the divisor summatory function is a function that is a sum over the divisor function. It frequently occurs in the study of the asymptotic behaviour of the Riemann zeta function. The various studies of the behaviour of the divisor function are sometimes called divisor problems.
The divisor summatory function is defined as
is the divisor function. The divisor function counts the number of ways that the integer n can be written as a product of two integers. More generally, one defines
where dk(n) counts the number of ways that n can be written as a product of k numbers. This quantity can be visualized as the count of the number of lattice points fenced off by a hyperbolic surface in k dimensions. Thus, for k=2, D(x)=D2(x) counts the number of points on a square lattice bounded on the left by the vertical-axis, on the bottom by the horizontal-axis, and to the upper-right by the hyperbola jk = x. Roughly, this shape may be envisioned as a hyperbolic simplex. This allows us to provide an alternative expression for D(x), and a simple way to compute it in time:
- , where
If the hyperbola in this context is replaced by a circle then determining the value of the resulting function is known as the Gauss circle problem.
Dirichlet's divisor problem
Finding a closed form for this summed expression seems to be beyond the techniques available, but it is possible to give approximations. The leading behaviour of the series is not difficult to obtain. Peter Gustav Lejeune Dirichlet demonstrated that
where is the Euler-Mascheroni constant, and the non-leading term is
Here, denotes Big-O notation. The Dirichlet divisor problem, precisely stated, is to find the smallest value of for which
holds true, for any . As of 2006[update], this problem remains unsolved. Progress has been slow. Many of the same methods work for this problem and for Gauss's circle problem, another lattice-point counting problem. Section F1 of Unsolved Problems in Number Theory  surveys what is known and not known about these problems.
- In 1904, G. Voronoi proved that the error term can be improved to :381
- In 1916, G.H. Hardy showed that . In particular, he demonstrated that for some constant , there exist values of x for which and values of x for which .:69
- In 1922, J. van der Corput improved Dirichlet's bound to :381
- In 1928, J. van der Corput proved that :381
- In 1950, Chih Tsung-tao and independently in 1953 H. E. Richert proved that :381
- In 1969, Grigori Kolesnik demonstrated that .:381
- In 1973, Grigori Kolesnik demonstrated that .:381
- In 1982, Grigori Kolesnik demonstrated that .:381
- In 1988, H. Iwaniec and C. J. Mozzochi proved that 
- In 2003, M.N. Huxley improved this to show that 
So, the true value of lies somewhere between 1/4 and 131/416 (approx. 0.3149); it is widely conjectured to be exactly 1/4. Theoretical evidence lends credence to this conjecture, since has a (non-Gaussian) limiting distribution. The value of 1/4 would also follow from a conjecture on exponent pairs.
Piltz divisor problem
In the generalized case, one has
where is a polynomial of degree . Using simple estimates, it is readily shown that
for integer . As in the case, the infimum of the bound is not known for any value of . Computing these infima is known as the Piltz divisor problem, after the name of the German mathematician Adolf Piltz (also see his German page). Defining the order as the smallest value for which holds, for any , one has the following results (note that is the of the previous section):
- E.C. Titchmarsh conjectures that
Both portions may be expressed as Mellin transforms:
for . Here, is the Riemann zeta function. Similarly, one has
and likewise for , for .
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