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Difference between revisions of "Riesz summation method"

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A method for summing series of numbers and functions; denoted by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823001.png" />. A series <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823002.png" /> is summable by the Riesz summation method <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823003.png" /> to the sum <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823004.png" /> if
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A method for summing series of numbers and functions; denoted by $(R,\lambda,k)$. A series $\sum_{n=0}^\infty a_n$ is summable by the Riesz summation method $(R,\lambda,k)$ to the sum $s$ if
  
<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/r082/r082300/r0823005.png" /></td> </tr></table>
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$$\lim_{\omega\to+\infty}\sum_{\lambda_n\leq\omega}\left(1-\frac{\lambda_n}{\omega}\right)^ka_n=s,$$
  
where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823006.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823007.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823008.png" /> is a continuous parameter. The method was introduced by M. Riesz [[#References|[1]]] for the summation of [[Dirichlet series|Dirichlet series]]. The method <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r0823009.png" /> is regular; when <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230010.png" /> it is equivalent to the Cesàro summation method <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230011.png" /> (cf. [[Cesàro summation methods|Cesàro summation methods]]), and these methods are compatible (cf. [[Compatibility of summation methods|Compatibility of summation methods]]).
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where $k>0$, $0\leq\lambda_0<\ldots<\lambda_n\to\infty$, and $\omega$ is a continuous parameter. The method was introduced by M. Riesz [[#References|[1]]] for the summation of [[Dirichlet series|Dirichlet series]]. The method $(R,\lambda,k)$ is regular; when $\lambda_n=n$ it is equivalent to the Cesàro summation method $(C,k)$ (cf. [[Cesàro summation methods|Cesàro summation methods]]), and these methods are compatible (cf. [[Compatibility of summation methods|Compatibility of summation methods]]).
  
Riesz considered also a method in which summability of the series <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230012.png" /> is defined by means of the limit of the sequence <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230013.png" />, where
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Riesz considered also a method in which summability of the series $\sum_{n=0}^\infty a_n$ is defined by means of the limit of the sequence $\{\sigma_m\}$, where
  
<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/r082/r082300/r08230014.png" /></td> </tr></table>
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$$\sigma_m=\frac{1}{P_m}\sum_{k=0}^mp_ks_k,$$
  
<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/r082/r082300/r08230015.png" /></td> </tr></table>
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$$P_m=\sum_{k=0}^mp_k\neq0,\quad s_k=\sum_{n=0}^ka_n.$$
  
This method is denoted by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230016.png" />. The method <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230017.png" /> is a modification of the method <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230018.png" /> (when <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230019.png" />) and is a generalization of it to the case of an arbitrary <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/r/r082/r082300/r08230020.png" />.
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This method is denoted by $(R,p_n)$. The method $(R,\lambda,k)$ is a modification of the method $(R,p_n)$ (when $k=1$) and is a generalization of it to the case of an arbitrary $k>0$.
  
 
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Revision as of 17:01, 18 September 2014

A method for summing series of numbers and functions; denoted by $(R,\lambda,k)$. A series $\sum_{n=0}^\infty a_n$ is summable by the Riesz summation method $(R,\lambda,k)$ to the sum $s$ if

$$\lim_{\omega\to+\infty}\sum_{\lambda_n\leq\omega}\left(1-\frac{\lambda_n}{\omega}\right)^ka_n=s,$$

where $k>0$, $0\leq\lambda_0<\ldots<\lambda_n\to\infty$, and $\omega$ is a continuous parameter. The method was introduced by M. Riesz [1] for the summation of Dirichlet series. The method $(R,\lambda,k)$ is regular; when $\lambda_n=n$ it is equivalent to the Cesàro summation method $(C,k)$ (cf. Cesàro summation methods), and these methods are compatible (cf. Compatibility of summation methods).

Riesz considered also a method in which summability of the series $\sum_{n=0}^\infty a_n$ is defined by means of the limit of the sequence $\{\sigma_m\}$, where

$$\sigma_m=\frac{1}{P_m}\sum_{k=0}^mp_ks_k,$$

$$P_m=\sum_{k=0}^mp_k\neq0,\quad s_k=\sum_{n=0}^ka_n.$$

This method is denoted by $(R,p_n)$. The method $(R,\lambda,k)$ is a modification of the method $(R,p_n)$ (when $k=1$) and is a generalization of it to the case of an arbitrary $k>0$.

References

[1] M. Riesz, "Une méthode de sommation équivalente à la méthode des moyennes arithmétique" C.R. Acad. Sci. Paris , 152 (1911) pp. 1651–1654
[2] F. Riesz, "Sur la sommation des séries de Dirichlet" C.R. Acad. Sci. Paris , 149 (1909) pp. 18–21
[3] G.H. Hardy, M. Riesz, "The general theory of Dirichlet series" , Cambridge Univ. Press (1915)
[4] G.H. Hardy, "Divergent series" , Clarendon Press (1949)


Comments

References

[a1] K. Zeller, W. Beekmann, "Theorie der Limitierungsverfahren" , Springer (1970)
How to Cite This Entry:
Riesz summation method. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Riesz_summation_method&oldid=33312
This article was adapted from an original article by I.I. Volkov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article