In mathematics , the Riesz mean is a certain mean of an arithmetic series . They were introduced by Marcel Riesz in 1911 as an improvement over the Cesàro mean [1] [2] . The Riesz mean should not be confused with the Bochner-Riesz mean or the Strong-Riesz mean .
Definition
Given a series
{
s
n
}
{\displaystyle \{s_{n}\}}
, the Riesz mean of the series is defined by
s
δ
(
λ
)
=
∑
n
≤
λ
(
1
−
n
λ
)
δ
s
n
{\displaystyle s^{\delta }(\lambda )=\sum _{n\leq \lambda }\left(1-{\frac {n}{\lambda }}\right)^{\delta }s_{n}}
Sometimes, a generalized Riesz mean is defined as
R
n
=
1
λ
n
∑
k
=
0
n
(
λ
k
−
λ
k
−
1
)
δ
s
k
{\displaystyle R_{n}={\frac {1}{\lambda _{n}}}\sum _{k=0}^{n}(\lambda _{k}-\lambda _{k-1})^{\delta }s_{k}}
Here, the
λ
n
{\displaystyle \lambda _{n}}
are sequence with
λ
n
→
∞
{\displaystyle \lambda _{n}\to \infty }
and with
λ
n
+
1
/
λ
n
→
1
{\displaystyle \lambda _{n+1}/\lambda _{n}\to 1}
as
n
→
∞
{\displaystyle n\to \infty }
. Other than this, the
λ
n
{\displaystyle \lambda _{n}}
are otherwise taken as arbitrary.
Riesz means are often used to explore the summability of sequences; typical summability theorems discuss the case of
s
n
=
∑
k
=
0
n
a
n
{\displaystyle s_{n}=\sum _{k=0}^{n}a_{n}}
for some sequence
{
a
n
}
{\displaystyle \{a_{n}\}}
. Typically, a sequence is summable when the limit
lim
n
→
∞
R
n
{\displaystyle \lim _{n\to \infty }R_{n}}
exists, or the limit
lim
δ
→
1
,
λ
→
∞
s
δ
(
λ
)
{\displaystyle \lim _{\delta \to 1,\lambda \to \infty }s^{\delta }(\lambda )}
exists, although the precise summability theorems in question often impose additional conditions.
Special cases
Let
a
n
=
1
{\displaystyle a_{n}=1}
for all
n
{\displaystyle n}
. Then
∑
n
≤
λ
(
1
−
n
λ
)
δ
=
1
2
π
i
∫
c
−
i
∞
c
+
i
∞
Γ
(
1
+
δ
)
Γ
(
s
)
Γ
(
1
+
δ
+
s
)
ζ
(
s
)
λ
s
d
s
=
λ
1
+
δ
+
∑
n
b
n
λ
−
n
{\displaystyle \sum _{n\leq \lambda }\left(1-{\frac {n}{\lambda }}\right)^{\delta }={\frac {1}{2\pi i}}\int _{c-i\infty }^{c+i\infty }{\frac {\Gamma (1+\delta )\Gamma (s)}{\Gamma (1+\delta +s)}}\zeta (s)\lambda ^{s}ds={\frac {\lambda }{1+\delta }}+\sum _{n}b_{n}\lambda ^{-n}}
Here, one must take
c
>
1
{\displaystyle c>1}
;
Γ
(
s
)
{\displaystyle \Gamma (s)}
is the Gamma function and
ζ
(
s
)
{\displaystyle \zeta (s)}
is the Riemann zeta function . The power series
∑
n
b
n
λ
−
n
{\displaystyle \sum _{n}b_{n}\lambda ^{-n}}
can be shown to be convergent for
λ
>
1
{\displaystyle \lambda >1}
. Note that the integral is of the form of an inverse Mellin transform .
Another interesting case connected with number theory arises by taking
a
n
=
Λ
(
n
)
{\displaystyle a_{n}=\Lambda (n)}
where
Λ
(
n
)
{\displaystyle \Lambda (n)}
is the Von Mangoldt function . Then
∑
n
≤
λ
(
1
−
n
λ
)
δ
Λ
(
n
)
=
−
1
2
π
i
∫
c
−
i
∞
c
+
i
∞
Γ
(
1
+
δ
)
Γ
(
s
)
Γ
(
1
+
δ
+
s
)
ζ
′
(
s
)
ζ
(
s
)
λ
s
d
s
=
λ
1
+
δ
+
∑
ρ
Γ
(
1
+
δ
)
Γ
(
ρ
)
Γ
(
1
+
δ
+
ρ
)
+
∑
n
c
n
λ
−
n
{\displaystyle \sum _{n\leq \lambda }\left(1-{\frac {n}{\lambda }}\right)^{\delta }\Lambda (n)=-{\frac {1}{2\pi i}}\int _{c-i\infty }^{c+i\infty }{\frac {\Gamma (1+\delta )\Gamma (s)}{\Gamma (1+\delta +s)}}{\frac {\zeta ^{\prime }(s)}{\zeta (s)}}\lambda ^{s}ds={\frac {\lambda }{1+\delta }}+\sum _{\rho }{\frac {\Gamma (1+\delta )\Gamma (\rho )}{\Gamma (1+\delta +\rho )}}+\sum _{n}c_{n}\lambda ^{-n}}
Again, one must take
c
>
1
{\displaystyle c>1}
. The sum over
ρ
{\displaystyle \rho }
is the sum over the zeroes of the Riemann zeta function, and
∑
n
c
n
λ
−
n
{\displaystyle \sum _{n}c_{n}\lambda ^{-n}}
is convergent for
λ
>
1
{\displaystyle \lambda >1}
.
The integrals that occur here are similar to the Nörlund-Rice integral ; very roughly, they can be connected to that integral via Perron's formula .
References
^ M. Riesz, Comptes Rendus , 12 June 1911
^ G.H. Hardy and J.E. Littlewood, "Contributions to the Theory of the Riemann Zeta-Function and the Theory of the Distribution of Primes", Acta Mathematica , 41 (1916) pp.119-196.