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A generalization of the improper Riemann integral on the class of functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466101.png" /> whose set of unboundedness points <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466102.png" /> has Jordan measure zero, and which are Riemann integrable on any segment not containing points of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466103.png" />. Let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466104.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466105.png" />, be a finite system of intervals containing <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466106.png" />. The Harnack integral is then defined by the equation
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<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466107.png" /></td> </tr></table>
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if the last limit for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466108.png" /> exists. The integral was introduced by A. Harnack [[#References|[1]]]. The condition that each interval <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h0466109.png" /> should have non-empty intersection with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h046/h046610/h04661010.png" /> was added to the original definition at a later date. As a result, the Harnack integral usually becomes conditionally convergent. It partly overlaps the [[Lebesgue integral|Lebesgue integral]] and is covered by the [[Perron integral|Perron integral]] and by the [[Denjoy integral|Denjoy integral]]. Nowadays, the Harnack integral is only of methodical and historical interest.
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A generalization of the improper Riemann integral on the class of functions  $  f $
 +
whose set of unboundedness points  $  E _ {f} $
 +
has Jordan measure zero, and which are Riemann integrable on any segment not containing points of  $  E _ {f} $.
 +
Let  $  \Delta _ {i} $,
 +
$  i = 1 \dots n $,
 +
be a finite system of intervals containing  $  E _ {f} $.
 +
The Harnack integral is then defined by the equation
 +
 
 +
$$
 +
( H)  \int\limits _ { a } ^ { b }  f ( x)  dx  = \
 +
\lim\limits  ( R)  \int\limits _ {( a, b) \setminus  \cup _ {i} \Delta _ {i} }
 +
f ( x)  dx,
 +
$$
 +
 
 +
if the last limit for $  \mathop{\rm mes}  ( \cup _ {i} \Delta _ {i} ) \rightarrow 0 $
 +
exists. The integral was introduced by A. Harnack [[#References|[1]]]. The condition that each interval $  \Delta _ {i} $
 +
should have non-empty intersection with $  E _ {f} $
 +
was added to the original definition at a later date. As a result, the Harnack integral usually becomes conditionally convergent. It partly overlaps the [[Lebesgue integral|Lebesgue integral]] and is covered by the [[Perron integral|Perron integral]] and by the [[Denjoy integral|Denjoy integral]]. Nowadays, the Harnack integral is only of methodical and historical interest.
  
 
====References====
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  A. Harnack,  "Anwendung der Fourier'schen Reihe auf die Theorie der Funktionen einer komplexen Veränderlichen"  ''Math. Ann.'' , '''21'''  (1883)  pp. 305–326</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  I.N. Pesin,  "Classical and modern integration theories" , Acad. Press  (1970)  (Translated from Russian)</TD></TR></table>
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  A. Harnack,  "Anwendung der Fourier'schen Reihe auf die Theorie der Funktionen einer komplexen Veränderlichen"  ''Math. Ann.'' , '''21'''  (1883)  pp. 305–326</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  I.N. Pesin,  "Classical and modern integration theories" , Acad. Press  (1970)  (Translated from Russian)</TD></TR></table>
 
 
  
 
====Comments====
 
====Comments====
 
  
 
====References====
 
====References====
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  E.W. Hobson,  "The theory of functions of a real variable and the theory of Fourier's series" , '''1''' , Dover, reprint  (1957)</TD></TR></table>
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  E.W. Hobson,  "The theory of functions of a real variable and the theory of Fourier's series" , '''1''' , Dover, reprint  (1957)</TD></TR></table>

Latest revision as of 19:43, 5 June 2020


A generalization of the improper Riemann integral on the class of functions $ f $ whose set of unboundedness points $ E _ {f} $ has Jordan measure zero, and which are Riemann integrable on any segment not containing points of $ E _ {f} $. Let $ \Delta _ {i} $, $ i = 1 \dots n $, be a finite system of intervals containing $ E _ {f} $. The Harnack integral is then defined by the equation

$$ ( H) \int\limits _ { a } ^ { b } f ( x) dx = \ \lim\limits ( R) \int\limits _ {( a, b) \setminus \cup _ {i} \Delta _ {i} } f ( x) dx, $$

if the last limit for $ \mathop{\rm mes} ( \cup _ {i} \Delta _ {i} ) \rightarrow 0 $ exists. The integral was introduced by A. Harnack [1]. The condition that each interval $ \Delta _ {i} $ should have non-empty intersection with $ E _ {f} $ was added to the original definition at a later date. As a result, the Harnack integral usually becomes conditionally convergent. It partly overlaps the Lebesgue integral and is covered by the Perron integral and by the Denjoy integral. Nowadays, the Harnack integral is only of methodical and historical interest.

References

[1] A. Harnack, "Anwendung der Fourier'schen Reihe auf die Theorie der Funktionen einer komplexen Veränderlichen" Math. Ann. , 21 (1883) pp. 305–326
[2] I.N. Pesin, "Classical and modern integration theories" , Acad. Press (1970) (Translated from Russian)

Comments

References

[a1] E.W. Hobson, "The theory of functions of a real variable and the theory of Fourier's series" , 1 , Dover, reprint (1957)
How to Cite This Entry:
Harnack integral. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Harnack_integral&oldid=47188
This article was adapted from an original article by V.A. Skvortsov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article