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A generalization of the concept of a [[Cauchy integral|Cauchy integral]] to a certain class of discontinuous functions; introduced by B. Riemann (1853). Consider a function $f$ which is given on an interval $[a,b]$. Let $a=x_0<x_1<\dots<x_n=b$ is a partition (subdivision) of the interval $[a,b]$ and $\Delta x_i = x_i-x_{i-1}$, where $i=1,\dots,n$. The sum
+
A generalization of the concept of a [[Cauchy integral]] to a certain class of discontinuous functions; introduced by B. Riemann (1853). Consider a function $f$ which is given on an interval $[a,b]$. Let $a=x_0<x_1<\dots<x_n=b$ is a partition (subdivision) of the interval $[a,b]$ and $\Delta x_i = x_i-x_{i-1}$, where $i=1,\dots,n$. The sum
 
\begin{equation}\label{eq:1}
 
\begin{equation}\label{eq:1}
 
\sigma = f(\xi_1)\Delta x_1+\dots+f(\xi x_i)\Delta x_i +\dots +f(x_n)\Delta x_n,
 
\sigma = f(\xi_1)\Delta x_1+\dots+f(\xi x_i)\Delta x_i +\dots +f(x_n)\Delta x_n,
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==Properties of the Riemann integral.==
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==Properties of the Riemann integral==
  
  
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# If two functions $f$ and $g$ are integrable over $[a,b]$ and if $f(x)\geqslant g(x)$ for every $x$ in this interval, then \begin{equation} \int\limits_a^bf(x)\,dx \geqslant \int\limits_a^bg(x)\,dx. \end{equation}
 
# If two functions $f$ and $g$ are integrable over $[a,b]$ and if $f(x)\geqslant g(x)$ for every $x$ in this interval, then \begin{equation} \int\limits_a^bf(x)\,dx \geqslant \int\limits_a^bg(x)\,dx. \end{equation}
 
# The integrability of a function $f$ over $[a,b]$ implies that the function $|f|$ is integrable over this interval, and the estimate \begin{equation} \left|\int\limits_a^bf(x)\,dx\right| \leqslant \int\limits_a^b|f(x)|\,dx \end{equation} holds.
 
# The integrability of a function $f$ over $[a,b]$ implies that the function $|f|$ is integrable over this interval, and the estimate \begin{equation} \left|\int\limits_a^bf(x)\,dx\right| \leqslant \int\limits_a^b|f(x)|\,dx \end{equation} holds.
# The mean-value formula: If two real-valued functions $f$ and $g$ are integrable over $[a,b]$, if the function $g$is non-negative or non-positive everywhere on this interval, and if $M$ and $m$ are the least upper and greatest lower bounds of $f$ on $[a,b]$, then a number $\mu$ can be found, $m\leqslant\mu\leqslant M$, such that the formula
+
# The mean-value formula: If two real-valued functions $f$ and $g$ are integrable over $[a,b]$, if the function $g$ is non-negative or non-positive everywhere on this interval, and if $M$ and $m$ are the least upper and greatest lower bounds of $f$ on $[a,b]$, then a number $\mu$ can be found, $m\leqslant\mu\leqslant M$, such that the formula \begin{equation}\label{eq:3} \int\limits_a^bf(x)g(x)\,dx = \mu\int\limits_a^bg(x)\,dx, \end{equation} holds. If, in addition, $f$ is continuous on $[a,b]$, then this interval will contain a point $\xi$ such that in formula \eqref{eq:3}, \begin{equation} \mu = f(\xi). \end{equation}
 
+
# The second mean-value formula (Bonnet's formula): If a function $f$ is real-valued and integrable over $[a,b]$ and if a function $g$ is real-valued and monotone on this interval, then a point $\xi$ can be found in $[a,b]$ such that the formula \begin{equation} \int\limits_a^bf(x)g(x)\,dx = g(a)\int\limits_a^{\xi}f(x)\,dx + g(b)\int\limits_{\xi}^bf(x)\,dx, \end{equation} holds.
<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/r/r081/r081950/r08195075.png" /></td> <td valign="top" style="width:5%;text-align:right;">(3)</td></tr></table>
 
 
 
holds. If, in addition, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195076.png" /> is continuous on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195077.png" />, then this interval will contain a point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195078.png" /> such that in formula (3),
 
 
 
<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/r/r081/r081950/r08195079.png" /></td> </tr></table>
 
 
 
8) The second mean-value formula (Bonnet's formula): If a function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195080.png" /> is real-valued and integrable over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195081.png" /> and if a function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195082.png" /> is real-valued and monotone on this interval, then a point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195083.png" /> can be found in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195084.png" /> such that the formula
 
 
 
<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/r/r081/r081950/r08195085.png" /></td> </tr></table>
 
 
 
holds.
 
 
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  B. Riemann,  "Ueber die Darstellbarkeit einer Function durch eine trigonometrische Reihe"  H. Weber (ed.) , ''B. Riemann's Gesammelte Mathematische Werke'' , Dover, reprint  (1953) pp. 227–271  ((Original: Göttinger Akad. Abh. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r081/r081950/r08195086.png" /> (1868)))</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  V.A. Il'in,   E.G. Poznyak,  "Fundamentals of mathematical analysis" , '''1–2''' , MIR  (1982) (Translated from Russian)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  L.D. Kudryavtsev,   "A course in mathematical analysis" , '''1–2''' , Moscow  (1988)  (In Russian)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  S.M. Nikol'skii,  "A course of mathematical analysis" , '''1–2''' , MIR  (1977)  (Translated from Russian)</TD></TR></table>
 
 
 
 
 
 
 
====Comments====
 
 
 
  
 
====References====
 
====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  G.E. Shilov,  "Mathematical analysis" , '''1–2''' , M.I.T.  (1974)  (Translated from Russian)</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  I.N. Pesin,  "Classical and modern integration theories" , Acad. Press  (1970)  (Translated from Russian)</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  K.R. Stromberg,  "Introduction to classical real analysis" , Wadsworth  (1981)</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  W. Rudin,  "Principles of mathematical analysis" , McGraw-Hill  (1976)  pp. 75–78</TD></TR></table>
+
<table>
 +
<TR><TD valign="top">[1]</TD> <TD valign="top">  B. Riemann,  "Ueber die Darstellbarkeit einer Function durch eine trigonometrische Reihe"  H. Weber (ed.) , ''B. Riemann's Gesammelte Mathematische Werke'' , Dover, reprint  (1953)  pp. 227–271  ((Original: Göttinger Akad. Abh. 13 (1868)))</TD></TR>
 +
<TR><TD valign="top">[2]</TD> <TD valign="top">  V.A. Il'in,  E.G. Poznyak,  "Fundamentals of mathematical analysis" , '''1–2''' , MIR  (1982)  (Translated from Russian)</TD></TR>
 +
<TR><TD valign="top">[3]</TD> <TD valign="top">  L.D. Kudryavtsev,  "A course in mathematical analysis" , '''1–2''' , Moscow  (1988)  (In Russian)</TD></TR>
 +
<TR><TD valign="top">[4]</TD> <TD valign="top">  S.M. Nikol'skii,  "A course of mathematical analysis" , '''1–2''' , MIR  (1977)  (Translated from Russian)</TD></TR>
 +
<TR><TD valign="top">[a1]</TD> <TD valign="top">  G.E. Shilov,  "Mathematical analysis" , '''1–2''' , M.I.T.  (1974)  (Translated from Russian)</TD></TR>
 +
<TR><TD valign="top">[a2]</TD> <TD valign="top">  I.N. Pesin,  "Classical and modern integration theories" , Acad. Press  (1970)  (Translated from Russian)</TD></TR>
 +
<TR><TD valign="top">[a3]</TD> <TD valign="top">  K.R. Stromberg,  "Introduction to classical real analysis" , Wadsworth  (1981)</TD></TR>
 +
<TR><TD valign="top">[a4]</TD> <TD valign="top">  W. Rudin,  "Principles of mathematical analysis" , McGraw-Hill  (1976)  pp. 75–78</TD></TR>
 +
</table>

Revision as of 08:13, 25 April 2016

A generalization of the concept of a Cauchy integral to a certain class of discontinuous functions; introduced by B. Riemann (1853). Consider a function $f$ which is given on an interval $[a,b]$. Let $a=x_0<x_1<\dots<x_n=b$ is a partition (subdivision) of the interval $[a,b]$ and $\Delta x_i = x_i-x_{i-1}$, where $i=1,\dots,n$. The sum \begin{equation}\label{eq:1} \sigma = f(\xi_1)\Delta x_1+\dots+f(\xi x_i)\Delta x_i +\dots +f(x_n)\Delta x_n, \end{equation} where $x_{i-1}\leq\xi_i\leq x_i$, is called the Riemann sum corresponding to the given partition of $[a,b]$ by the points $x_i$ and to the sample of points $\xi_i$. The number $I$ is called the limit of the Riemann sums \eqref{eq:1} as $\max_i \Delta x_i \to 0$ if for any $\varepsilon>0$ a $\delta>0$ can be found such that $\max_i \Delta x_i < \delta$ implies the inequality $|\sigma - I|<\varepsilon$. If the Riemann sums have a finite limit $I$ as $\max_i \Delta x_i \to 0$, then the function $f$ is called Riemann integrable over $[a,b]$, where $a< b$. The limit is known as the definite Riemann integral of $f$ over $[a,b]$, and is written as \begin{equation}\label{eq:2} \int\limits_a^bf(x)\,dx. \end{equation} When $a=b$ then, by definition, \begin{equation} \int\limits_a^af(x)\,dx = 0, \end{equation} and when $a>b$ the integral \eqref{eq:2} is defined using the equation \begin{equation} \int\limits_a^bf(x)\,dx = -\int\limits_b^af(x)\,dx. \end{equation} A necessary and sufficient condition for the Riemann integrability of $f$ over $[a,b]$ is the boundedness of $f$ on this interval and the zero value of the Lebesgue measure of the set of all points of discontinuity of $f$ contained in $[a,b]$.



Properties of the Riemann integral

  1. Every Riemann-integrable function $f$ on $[a,b]$ is also bounded on this interval (the converse is not true: The Dirichlet function is an example of a bounded and non-integrable function on $[a,b]$).
  2. The linearity property: For any constants $\alpha$ and $\beta$, the integrability over $[a,b]$ of both functions $f$ and $g$ implies that the function $\alpha f + \beta g$ is integrable over this interval, and the equation \begin{equation} \int\limits_a^b[\alpha f(x) + \beta g(x)]\,dx = \alpha\int\limits_a^bf(x)\,dx + \beta\int\limits_a^bg(x)\,dx \end{equation} holds.
  3. The integrability over $[a,b]$ of both functions $f$ and $g$ implies that their product $fg$ is integrable over this interval.
  4. Additivity: The integrability of a function $f$ over both intervals $[a,c]$ and $[c,b]$ implies that $f$ is integrable over $[a,b]$, and \begin{equation} \int\limits_a^bf(x)\,dx = \int\limits_a^cf(x)\,dx + \int\limits_c^bf(x)\,dx. \end{equation}
  5. If two functions $f$ and $g$ are integrable over $[a,b]$ and if $f(x)\geqslant g(x)$ for every $x$ in this interval, then \begin{equation} \int\limits_a^bf(x)\,dx \geqslant \int\limits_a^bg(x)\,dx. \end{equation}
  6. The integrability of a function $f$ over $[a,b]$ implies that the function $|f|$ is integrable over this interval, and the estimate \begin{equation} \left|\int\limits_a^bf(x)\,dx\right| \leqslant \int\limits_a^b|f(x)|\,dx \end{equation} holds.
  7. The mean-value formula: If two real-valued functions $f$ and $g$ are integrable over $[a,b]$, if the function $g$ is non-negative or non-positive everywhere on this interval, and if $M$ and $m$ are the least upper and greatest lower bounds of $f$ on $[a,b]$, then a number $\mu$ can be found, $m\leqslant\mu\leqslant M$, such that the formula \begin{equation}\label{eq:3} \int\limits_a^bf(x)g(x)\,dx = \mu\int\limits_a^bg(x)\,dx, \end{equation} holds. If, in addition, $f$ is continuous on $[a,b]$, then this interval will contain a point $\xi$ such that in formula \eqref{eq:3}, \begin{equation} \mu = f(\xi). \end{equation}
  8. The second mean-value formula (Bonnet's formula): If a function $f$ is real-valued and integrable over $[a,b]$ and if a function $g$ is real-valued and monotone on this interval, then a point $\xi$ can be found in $[a,b]$ such that the formula \begin{equation} \int\limits_a^bf(x)g(x)\,dx = g(a)\int\limits_a^{\xi}f(x)\,dx + g(b)\int\limits_{\xi}^bf(x)\,dx, \end{equation} holds.

References

[1] B. Riemann, "Ueber die Darstellbarkeit einer Function durch eine trigonometrische Reihe" H. Weber (ed.) , B. Riemann's Gesammelte Mathematische Werke , Dover, reprint (1953) pp. 227–271 ((Original: Göttinger Akad. Abh. 13 (1868)))
[2] V.A. Il'in, E.G. Poznyak, "Fundamentals of mathematical analysis" , 1–2 , MIR (1982) (Translated from Russian)
[3] L.D. Kudryavtsev, "A course in mathematical analysis" , 1–2 , Moscow (1988) (In Russian)
[4] S.M. Nikol'skii, "A course of mathematical analysis" , 1–2 , MIR (1977) (Translated from Russian)
[a1] G.E. Shilov, "Mathematical analysis" , 1–2 , M.I.T. (1974) (Translated from Russian)
[a2] I.N. Pesin, "Classical and modern integration theories" , Acad. Press (1970) (Translated from Russian)
[a3] K.R. Stromberg, "Introduction to classical real analysis" , Wadsworth (1981)
[a4] W. Rudin, "Principles of mathematical analysis" , McGraw-Hill (1976) pp. 75–78
How to Cite This Entry:
Riemann integral. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Riemann_integral&oldid=38635
This article was adapted from an original article by V.A. Il'in (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article