In the field of mathematical analysis, a general Dirichlet series is an infinite series that takes the form of
Contents
- Fundamental theorems
- Abscissa of convergence
- Abscissa of absolute convergence
- Analytic functions
- Further generalizations
- References
where
A simple observation shows that an 'ordinary' Dirichlet series
is obtained by substituting
is obtained when
Fundamental theorems
If a Dirichlet series is convergent at
and convergent for any
There are now three possibilities regarding the convergence of a Dirichlet series, i.e. it may converge for all, for none or for some values of s. In the latter case, there exist a
Abscissa of convergence
The abscissa of convergence of a Dirichlet series can be defined as
The line
The abscissa, line and half-plane of convergence of a Dirichlet series are analogous to radius, boundary and disk of convergence of a power series.
On the line of convergence, the question of convergence remains open as in the case of power series. However, if a Dirichlet series converges and diverges at different points on the same vertical line, then this line must be the line of convergence. The proof is implicit in the definition of abscissa of convergence. An example would be the series
which converges at
Suppose that a Dirichlet series does not converge at
If
If
Abscissa of absolute convergence
A Dirichlet series is absolutely convergent if the series
is convergent. As usual, an absolutely convergent Dirichlet series is convergent, but the converse is not always true.
If a Dirichlet series is absolutely convergent at
The abscissa of absolute convergence can be defined as
The line and half-plane of absolute convergence can be defined similarly. There are also two formulas to compute
If
If
In general, the abscissa of convergence does not coincide with abscissa of absolute convergence. Thus, there might be a strip between the line of convergence and absolute convergence where a Dirichlet series is conditionally convergent. The width of this strip is given by
In the case where L= 0, then
All the formulas provided so far still hold true for 'ordinary' Dirichlet series by substituting
Analytic functions
A function represented by a Dirichlet series
is analytic on the half-plane of convergence. Moreover, for
Further generalizations
A Dirichlet series can be further generalized to the multi-variable case where