Froda's theorem
In mathematics, Darboux-Froda's theorem, named after Alexandru Froda, a Romanian mathematician, describes the set of discontinuities of a monotone real-valued function of a real variable. Usually, this theorem appears in literature without a name. It was written in A. Froda' thesis in 1929 .[1][2]. As it is acknowledged in the thesis, it is in fact due Jean Gaston Darboux [3]
Definitions
- Consider a function f of real variable x with real values defined in a neighborhood of a point
and the function f is discontinuous at the point on the real axis
. We will call a removable discontinuity or a jump discontinuity a discontinuity of the first kind.[4]
- Denote
and
. Then if
and
are finite we will call the difference
the jump[5] of f at
.
If the function is continuous at then the jump at
is zero. Moreover, if
is not continuous at
, the jump can be zero at
if
.
Precise statement
Let f be a real-valued monotone function defined on an interval I. Then the set of discontinuities of the first kind is at most countable.
One can prove[6][7] that all points of discontinuity of a monotone real-valued function defined on an interval are jump discontinuities and hence, by our definition, of the first kind. With this remark Froda's theorem takes the stronger form:
Let f be a monotone function defined on an interval . Then the set of discontinuities is at most countable.
Proof
Let be an interval and
defined on
an increasing function. We have
for any . Let
and let
be
points inside
at which the jump of
is greater or equal to
:
We have or
.
Then
and hence: .
Since we have that the number of points at which the jump is greater than
is finite or zero.
We define the following sets:
,
We have that each set is finite or the empty set. The union
contains all points at which the jump is positive and hence contains all points of discontinuity. Since every
is at most countable, we have that
is at most countable.
If is decreasing the proof is similar.
If the interval is not closed and bounded (and hence by Heine–Borel theorem not compact) then the interval can be written as a countable union of closed and bounded intervals
with the property that any two consecutive intervals have an endpoint in common:
If then
where
is a strictly decreasing sequence such that
In a similar way if
or if
.
In any interval we have at most countable many points of discontinuity, and since a countable union of at most countable sets is at most countable, it follows that the set of all discontinuities is at most countable.
See also
Notes
- ↑ Alexandru Froda, Sur la Distribution des Propriétés de Voisinage des Fonctions de Variables Réelles, These, Harmann, Paris, 3 December 1929
- ↑ Alexandru Froda – Collected Papers (Opera Matematica), Vol.1, Ed. Academ. Romane, 2000
- ↑ Jean Gaston Darboux Mémoire sur les fonctions discontinues, Annales de l'École Normale supérieure, 2-ème série, t. IV, 1875, Chap VI.
- ↑ Walter Rudin, Principles of Mathematical Analysis, McGraw-Hill 1964, (Def. 4.26, pp. 81–82)
- ↑ M. Nicolescu, N. Dinculeanu, S. Marcus, Mathematical Analysis (Bucharest 1971), Vol.1, Pg.213, [in Romanian]
- ↑ W. Rudin, Principles of Mathematical Analysis, McGraw–Hill 1964 (Corollary, p.83)
- ↑ M. Nicolescu, N. Dinculeanu, S. Marcus, Mathematical Analysis (Bucharest 1971), Vol.1, Pg.213, [in Romanian]
References
- Bernard R. Gelbaum, John M. H. Olmsted, Counterexamples in Analysis, Holden–Day, Inc., 1964. (18. Page 28)
- John M. H. Olmsted, Real Variables, Appleton–Century–Crofts, Inc., New York (1956), (Page 59, Ex. 29).