Dirichlet series
Let be an increasing sequence of positive real numbers tending to . A Dirichlet series with exponents is a series of the form
where and all the are complex numbers.
An ordinary Dirichlet series is one having for all . It is written
The best-known examples are the Riemann zeta function (in which is the constant (http://planetmath.org/ConstantFunction) ) and the more general Dirichlet L-series (in which the mapping is multiplicative and periodic).
When , the Dirichlet series is just a power series in the variable .
The following are the basic convergence properties of Dirichlet series. There is nothing profound about their proofs, which can be found in [1] and in various other works on complex analysis and analytic number theory.
Let be a Dirichlet series.
-
1.
If converges at , then converges uniformly in the region
where is any real number such that . (Such a region is known as a “Stoltz angle”.)
-
2.
Therefore, if converges at , its sum defines a holomorphic function on the region , and moreover as within any Stoltz angle.
-
3.
identically if and only if all the coefficients are zero.
So, if converges somewhere but not everywhere in , then the domain of its convergence is the region for some real number , which is called the abscissa of convergence of the Dirichlet series. The abscissa of convergence of the series , if it exists, is called the abscissa of absolute convergence of .
Now suppose that the coefficients are all real and nonnegative. If the series converges for , and the resulting function admits an analytic extension (http://planetmath.org/AnalyticContinuation) to a neighbourhood of , then the series converges in a neighbourhood of . Consequently, the domain of convergence of (unless it is the whole of ) is bounded by a singularity at a point on the real axis.
Finally, return to the general case of any complex numbers , but suppose , so is an ordinary Dirichlet series .
-
1.
If the sequence is bounded, then converges absolutely in the region .
-
2.
If the partial sums are bounded, then converges (not necessarily absolutely) in the region .
References
- 1 Jean-Pierre Serre. A Course in Arithmetic, chapter VI. Springer-Verlag, 1973. http://www.emis.de/cgi-bin/zmen/ZMATH/en/quick.html?type=html&an=0256.12001Zbl 0256.12001.
- 2 E. C. Titchmarsh. The Theory of Functions. Oxford Univ. Press, second edition, 1958. http://www.emis.de/cgi-bin/zmen/ZMATH/en/quick.html?type=html&an=0336.30001Zbl 0336.30001.
Title | Dirichlet series |
Canonical name | DirichletSeries |
Date of creation | 2013-03-22 13:59:22 |
Last modified on | 2013-03-22 13:59:22 |
Owner | bbukh (348) |
Last modified by | bbukh (348) |
Numerical id | 8 |
Author | bbukh (348) |
Entry type | Definition |
Classification | msc 30B50 |
Related topic | DirichletLFunction |
Related topic | RiemannZetaFunction |
Related topic | DirichletLSeries |
Defines | ordinary Dirichlet series |
Defines | Stoltz angle |
Defines | abscissa of convergence |
Defines | abscissa of absolute convergence |