In
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, the Dirichlet function is the
indicator function 1
Q or
of the set of
rational numbers
In mathematics, a rational number is a number that can be expressed as the quotient or fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ). The set of all rat ...
Q, i.e. if ''x'' is a rational number and if ''x'' is not a rational number (i.e. an
irrational number
In mathematics, the irrational numbers (from in- prefix assimilated to ir- (negative prefix, privative) + rational) are all the real numbers that are not rational numbers. That is, irrational numbers cannot be expressed as the ratio of two inte ...
).
It is named after the mathematician
Peter Gustav Lejeune Dirichlet
Johann Peter Gustav Lejeune Dirichlet (; 13 February 1805 – 5 May 1859) was a German mathematician who made deep contributions to number theory (including creating the field of analytic number theory), and to the theory of Fourier series and ...
. It is an example of
pathological function which provides counterexamples to many situations.
Topological properties
- The Dirichlet function is nowhere continuous.
Its restrictions to the set of rational numbers and to the set of irrational numbers are constants and therefore continuous. The Dirichlet function is an archetypal example of the
Blumberg theorem
In mathematics, the Blumberg theorem states that for any real function f : \R \to \R there is a Dense set, dense subset D of \mathbb such that the Restriction_(mathematics), restriction of f to D is continuous function, continuous.
For instance, t ...
.
- The Dirichlet function can be constructed as the double pointwise limit of a sequence of continuous functions, as follows:
for integer ''j'' and ''k''. This shows that the Dirichlet function is a Baire class 2 function. It cannot be a Baire class 1 function because a Baire class 1 function can only be discontinuous on a
meagre set
In the mathematical field of general topology, a meagre set (also called a meager set or a set of first category) is a subset of a topological space that is small or negligible in a precise sense detailed below. A set that is not meagre is calle ...
.
Periodicity
For any real number ''x'' and any positive rational number ''T'', 1
Q(''x'' + ''T'') = 1
Q(''x''). The Dirichlet function is therefore an example of a real
periodic function
A periodic function is a function that repeats its values at regular intervals. For example, the trigonometric functions, which repeat at intervals of 2\pi radians, are periodic functions. Periodic functions are used throughout science to des ...
which is not
constant but whose set of periods, the set of rational numbers, is a
dense subset
In topology and related areas of mathematics, a subset ''A'' of a topological space ''X'' is said to be dense in ''X'' if every point of ''X'' either belongs to ''A'' or else is arbitrarily "close" to a member of ''A'' — for instance, the ra ...
of R.
Integration properties
- The Dirichlet function is not Riemann-integrable on any segment of R whereas it is bounded because the set of its discontinuity points is not negligible (for the Lebesgue measure).
- The Dirichlet function provides a counterexample showing that the
monotone convergence theorem
In the mathematical field of real analysis, the monotone convergence theorem is any of a number of related theorems proving the convergence of monotonic sequences (sequences that are non-decreasing or non-increasing) that are also bounded. Inform ...
is not true in the context of the Riemann integral.
- The Dirichlet function is Lebesgue-integrable on R and its integral over R is zero because it is zero except on the set of rational numbers which is negligible (for the Lebesgue measure).
See also
*
Thomae's function
Thomae's function is a real-valued function of a real variable that can be defined as:
f(x) =
\begin
\frac &\textx = \tfrac\quad (x \text p \in \mathbb Z \text q \in \mathbb N \text\\
0 &\textx \text
\end
It is named after Carl Jo ...
, a variation that is discontinuous only at the rational numbers
References
{{Reflist
Dirichlet
Johann Peter Gustav Lejeune Dirichlet (; 13 February 1805 – 5 May 1859) was a German mathematician who made deep contributions to number theory (including creating the field of analytic number theory), and to the theory of Fourier series and ...
Real analysis