# 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**.

## Definition[edit]

The divisor summatory function is defined as

where

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 *d*_{k}(*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*) = *D*_{2}(*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.

Sequence of D(n)(sequence A006218 in the OEIS):

0, 1, 3, 5, 8, 10, 14, 16, 20, 23, 27, 29, 35, 37, 41, 45, 50, 52, 58, 60, 66, 70, 74, 76, 84, 87, 91, 95, 101, 103, 111, ...

## Dirichlet's divisor problem[edit]

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 today, 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*
^{[1]}
surveys what is known and not known about these problems.

- In 1904, G. Voronoi proved that the error term can be improved to
^{[2]}^{: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 .^{[3]}^{:69} - In 1922, J. van der Corput improved Dirichlet's bound to .
^{[2]}^{:381} - In 1928, J. van der Corput proved that .
^{[2]}^{:381} - In 1950, Chih Tsung-tao and independently in 1953 H. E. Richert proved that .
^{[2]}^{:381} - In 1969, Grigori Kolesnik demonstrated that .
^{[2]}^{:381} - In 1973, Grigori Kolesnik demonstrated that .
^{[2]}^{:381} - In 1982, Grigori Kolesnik demonstrated that .
^{[2]}^{:381} - In 1988, H. Iwaniec and C. J. Mozzochi proved that .
^{[4]} - In 2003, M.N. Huxley improved this to show that .
^{[5]}

So, lies somewhere between 1/4 and 131/416 (approx. 0.3149); it is widely conjectured to be 1/4. Theoretical evidence lends credence to this conjecture, since has a (non-Gaussian) limiting distribution.^{[6]} The value of 1/4 would also follow from a conjecture on exponent pairs.^{[7]}

## Piltz divisor problem[edit]

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):

^{[5]}

^{[8]}and^{[9]}

- E. C. Titchmarsh conjectures that

## Mellin transform[edit]

Both portions may be expressed as Mellin transforms:

for . Here, is the Riemann zeta function. Similarly, one has

with . The leading term of is obtained by shifting the contour past the double pole at : the leading term is just the residue, by Cauchy's integral formula. In general, one has

and likewise for , for .

## Notes[edit]

**^**Guy, Richard K. (2004).*Unsolved Problems in Number Theory*(3rd ed.). Berlin: Springer. ISBN 978-0-387-20860-2.- ^
^{a}^{b}^{c}^{d}^{e}^{f}^{g}Ivic, Aleksandar (2003).*The Riemann Zeta-Function*. New York: Dover Publications. ISBN 0-486-42813-3. **^**Montgomery, Hugh; R. C. Vaughan (2007).*Multiplicative Number Theory I: Classical Theory*. Cambridge: Cambridge University Press. ISBN 978-0-521-84903-6.**^**Iwaniec, H.; C. J. Mozzochi (1988). "On the divisor and circle problems".*Journal of Number Theory*.**29**: 60–93. doi:10.1016/0022-314X(88)90093-5.- ^
^{a}^{b}Huxley, M. N. (2003). "Exponential sums and lattice points III".*Proc. London Math. Soc*.**87**(3): 591–609. doi:10.1112/S0024611503014485. ISSN 0024-6115. Zbl 1065.11079. **^**Heath-Brown, D. R. (1992). "The distribution and moments of the error term in the Dirichlet divisor problem".*Acta Arithmetica*.**60**(4): 389–415. doi:10.4064/aa-60-4-389-415. ISSN 0065-1036. S2CID 59450869.Theorem 1 The function has a distribution function

**^**Montgomery, Hugh L. (1994).*Ten lectures on the interface between analytic number theory and harmonic analysis*. Regional Conference Series in Mathematics.**84**. Providence, RI: American Mathematical Society. p. 59. ISBN 0-8218-0737-4. Zbl 0814.11001.**^**G. Kolesnik. On the estimation of multiple exponential sums, in "Recent Progress in Analytic Number Theory", Symposium Durham 1979 (Vol. 1), Academic, London, 1981, pp. 231–246.**^**Aleksandar Ivić. The Theory of the Riemann Zeta-function with Applications (Theorem 13.2). John Wiley and Sons 1985.

## References[edit]

- H.M. Edwards,
*Riemann's Zeta Function*, (1974) Dover Publications, ISBN 0-486-41740-9 - E. C. Titchmarsh,
*The theory of the Riemann Zeta-Function*, (1951) Oxford at the Clarendon Press, Oxford.*(See chapter 12 for a discussion of the generalized divisor problem)* - Apostol, Tom M. (1976),
*Introduction to analytic number theory*, Undergraduate Texts in Mathematics, New York-Heidelberg: Springer-Verlag, ISBN 978-0-387-90163-3, MR 0434929, Zbl 0335.10001*(Provides an introductory statement of the Dirichlet divisor problem.)* - H. E. Rose.
*A Course in Number Theory.*, Oxford, 1988. - M.N. Huxley (2003) 'Exponential Sums and Lattice Points III',
*Proc. London Math. Soc.*(3)87: 591–609