Möbius orthogonality in density for zero entropy dynamical systems

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Möbius orthogonality in density for zero entropy dynamical systems

Alexander Gomilko, Mariusz Lemańczyk, Thierry de la Rue

To cite this version:

Alexander Gomilko, Mariusz Lemańczyk, Thierry de la Rue. Möbius orthogonality in density for zero

entropy dynamical systems. Pure and Applied Functional Analysis, Yokohama Publishers, 2020, 5

(6), pp.1357-1376. �hal-02130224�

DYNAMICAL SYSTEMS

ALEXANDER GOMILKO, MARIUSZ LEMAŃCZYK, AND THIERRY DE LA RUE

Abstract. It is proved that whenever a zero entropy dynamical system pX, Tq has only countably many ergodic measures andµstands for the arithmetic Möbius function then there existsA“ApX, Tq ĂNof logarithmic density one such that for each fPCpXq,

AQNÑ8lim 1 N

ÿ

nďN

fpTⁿxqµpnq “0

uniformly in x P X, in particular, the density version of Möbius orthogonality holds.

1. Introduction

Following P. Sarnak [14], we say that a topological system p X, T q is Möbius orthog- onal if

(1) lim

NÑ8

1 N

ÿ

nďN

f p T

ⁿ

x q µ p n q “ 0

for all f P C p X q and x P X (here µ stands for the classical arithmetic Möbius function). By the standard trick of summation by parts (which we recall below for the reader’s convenience), we obtain that the Möbius orthogonality of p X, T q implies the logarithmic Möbius orthogonality of p X, T q :

(2) lim

NÑ8

1 log N

ÿ

nďN

1

n f p T

ⁿ

x q µ p n q “ 0

for all f P C p X q and x P X. The celebrated Sarnak’s conjecture [14] claims that all zero entropy systems are Möbius orthogonal, but this statement has been established only for some selected classes (we refer the reader to the bibliography in survey [9] to see for which classes). In some contrast to this, a considerable progress has been made recently in our understanding of the logarithmic Sarnak’s conjecture: Frantzikinakis and Host [10] proved that each zero entropy system whose set of ergodic measures is countable is logarithmic Möbius orthogonal. Earlier, Tao [17] proved that the logarithmic Sarnak’s conjecture is equivalent to the logarithmic version of the classical Chowla conjecture (from 1965) on auto-correlations of the Möbius function.

¹

While it looked rather odd to expect that we can say anything interesting about Cesàro type averaging knowing the convergence of logarithmic averages, it has been proved in [12] that the logarithmic Chowla conjecture implies the validity of the Chowla conjecture along a subsequence. In fact, the result was a consequence of some general mechanism when a certain sequence (in a locally convex space) of logarithmic

1Sarnak’s conjecture itself was motivated by the fact that the Chowla conjecture implies Sarnak’s conjecture [3], [14], [15]. See also [16], [19], [20], where special cases of the validity of the logarithmic Chowla conjecture have been proved.

averages

²

converges to an extremal point. This approach seems to fail if (what per- haps is natural), we would like to prove that the logarithmic Möbius orthogonality of a fixed system implies its Möbius orthogonality along a subsequence. However, commenting on [12], Tao [18] was able to prove a stronger result using a different method (second moment type argument). Namely, he proved that if the logarithmic Chowla conjecture holds then the Chowla conjecture holds along a subsequence of full logarithmic density; in particular of upper density 1.

The aim of this note is to show how to adapt Tao’s argument (this is done in Theorem 2.1 below) to be able to apply it to systems satisfying some (seemingly) stronger condition than the logarithmic Möbius orthogonality and which allows one to deduce Möbius orthogonality in full logarithmic density. In order to formulate such a result we first recall the strong MOMO notion introduced in [3]. Namely, a system p X, T q satisfies this property if for all increasing sequences p b

_k

q Ă N with b

_k`1

´ b

k

Ñ 8 , all p x

k

q Ă X and f P C p X q , we have

(3) 1

b

_K`1

ÿ

kďK

ˇ ˇ ˇ ˇ ˇ ˇ

ÿ

b_kďnăbk`1

f p T

^n´bk

x

_k

q µ p n q ˇ ˇ ˇ ˇ ˇ ˇ ÝÝÝÝÑ

KÑ8

0,

while if under the same assumptions we have

(4) 1

log b

_K`1

ÿ

kďK

ˇ ˇ ˇ ˇ ˇ ˇ

ÿ

bkďnăbk`1

1

n f p T

^n´bk

x

_k

q µ p n q ˇ ˇ ˇ ˇ ˇ ˇ ÝÝÝÝÑ

KÑ8

0,

then we say that p X, T q satisfies the logarithmic strong MOMO property. It has been proved in [3] that Sarnak’s conjecture is equivalent to the fact that all zero entropy systems enjoy the strong MOMO property.

Note that (3) is equivalent to

(5) lim

KÑ8

1 b

_K`1

ÿ

kďK

› ›

› ›

› › ÿ

bkďnăbk`1

µ p n q f ˝ T

ⁿ

› ›

› ›

› ›

CpXq

“ 0,

and (4) is equivalent to

(6) lim

KÑ8

1 log b

_K`1

ÿ

kďK

› ›

› ›

› › ÿ

bkďnăb_k`1

µ p n q n f ˝ T

ⁿ

› ›

› ›

› ›

CpXq

“ 0,

for all increasing sequences p b

_k

q Ă N with b

_k`1

´ b

_k

Ñ 8 and f P C p X q . From (5) and the triangular inequality we obtain

(7) lim

kÑ8

1 b

_k`1

› ›

› ›

› › ÿ

năb_k`1

µ p n q f ˝ T

ⁿ

› ›

› ›

› ›

CpXq

“ 0,

whenever b

_k`1

´ b

_k

Ñ 8 as k Ñ 8 . Then it is easy to deduce that (7) holds for b

_k

: “ k, that is, the uniform convergence in Möbius orthogonality (1) holds. Analogously, we

2The Chowla conjecture can be reformulated using the language of quasi-generic points for invari- ant measures in a certain shift space; it is then equivalent to the fact that that the empiric measures determined byµconverge to a certain natural measure which is ergodic, hence to an extremal point, see e.g. the survey [9]. Similarly, we deal with the logarithmic Chowla conjecture.

obtain that the logarithmic strong MOMO property implies the uniform convergence in the logarithmic Möbius orthogonality (2).

Here is our main result:

Theorem 1.1. Assume that a topological system p X, T q satisfies the logarithmic strong MOMO property. Then there exists A “ A p X, T q Ă N with full logarithmic density such that, for each f P C p X q ,

(8) lim

AQNÑ8

› ›

› ›

› 1 N

ÿ

nďN

µ p n q f ˝ T

ⁿ

› ›

› ›

›

CpXq

“ 0.

In particular, Möbius orthogonality holds along a subsequence (of N ) of full logarith- mic density.

One of the main results in [10] states that, if a system p X, T q has zero entropy and if its set of ergodic measures is countable, then the system is logarithmic Möbius orthogonal. We will show that such systems satisfy the strong logarithmic MOMO property, hence obtaining the following:

³

Corollary 1.2. Let p X, T q be a zero entropy ergodic dynamical system such that the set M

^e

p X, T q of ergodic T -invariant measures is countable. Then, there exists A “ A p X, T q Ă N with full logarithmic density along which Möbius orthogonality holds uniformly in x P X.

In particular, the above holds for all zero entropy uniquely ergodic systems. Corol- lary 1.2 is slightly surprising even for horocycle flows (in the cocompact case), where we know that Möbius orthogonality holds [5] but it is open (see questions in [9], [13]) whether Möbius orthogonality holds in its uniform form. By Corollary 1.2, we have that a uniform version holds along a subsequence of logarithmic density 1 (let alone the upper density of this subsequence is 1). A use of [11] shows that Corollary 1.2 remains valid when µ is replaced by any multiplicative function which is strongly aperiodic.

The rest of the note is devoted to give some illustrations how Theorem 2.1 (which is an adaptation of Tao’s result) can be applied in other situations. For example, we will show how in the main result in [12] we can achieve full logarithmic density. Besides, we note that in the classical Davenport-Erdös theorem on the existence of the logarithmic density [7] of sets of multiples, the upper asymptotic density is achieved along a set of full logarithmic density. Finally, we note in passing the logical implication: Chowla conjecture of order 2 ñ PNT along a subsequence of logarithmic density 1.

A few words on basic concepts and notation: Given a subset C Ă N , we de- note by δ p C q its logarithmic density: δ p C q : “ lim

_NÑ8

p 1 { log N q ř

nďN, nPC 1 n

, as- suming that the limit exists. It is classical that d p C q ď δ p C q ď d p C q , where d p C q : “ lim sup

_NÑ8N¹

|r 1, N s X C | stands for the upper asymptotic density, and similarly the lower (asymptotic) density d p C q is defined as the lim inf . In fact, these inequalities are direct consequences of the classical relationship between Cesàro aver- ages and harmonic averages: given a sequence p a

_n

q and setting s

_n

: “ ř

jďn

a

_j

, s

₀

: “ 0,

3We were informed by N. Frantzikinakis during the workshop “Sarnak’s Conjecture” at the Amer- ican Institute of Mathematics in mid-December 2018 that, independently of us, he can prove Corol- lary1.2by modifying some arguments in [10].

we have:

ÿ

1ďnďN

a

_n

n “ ÿ

1ďnďN

1

n p s

_n

´ s

_n´1

q “ ÿ

1ďnďN´1

s

_n

ˆ 1

n ´ 1 n ` 1

˙

` s

_N

N “ ÿ

nďN´1

s

_n

n

1

n ` 1 ` s

_N

N (9)

which basically says that the logarithmic averages of p a

_n

q are the logarithmic averages of Cesàro averages (sometimes, we only use the fact that the harmonic averages are convex combinations of Cesàro averages).

In what follows when we speak about subsequences of natural numbers, we always mean increasing sequences of natural numbers (so that subsequences are the same as infinite subsets). In Corollary 1.2, we find a subsequence of full logarithmic density which depends however on the system p X, T q . The methods used in this note do not seem to get one universal subsequence along which Sarnak’s conjecture (i.e. Möbius orthogonality for zero entropy systems) holds. We could get such a universal sequence (see Proposition 1.3 below) if we were able to prove Sarnak’s conjecture along a full logarithmic density sequence for each zero entropy system, that is, by [17], if the logarithmic Chowla conjecture holds. More precisely:

Proposition 1.3. Assume that for each zero entropy dynamical system p X, T q there exists a subsequence p N

_k

p X, T qq

^k

of natural numbers with δ pt N

_k

p X, T q : k ě 1 uq “ 1 such that

(10) lim

kÑ8

1 N

_k

p X, T q

ÿ

nďNkpX,Tq

f p T

ⁿ

x q µ p n q “ 0

for all f P C p X q and x P X. Then there exists a subsequence p N

_k

q of natural numbers, δ pt N

_k

: k ě 1 uq “ 1 such that for each zero entropy dynamical system p X, T q , (10) holds along p N

k

q .

To see the proof of Proposition 1.3, we have:

a) By assumption and the classical Lemma 2.5, we obtain that for each zero entropy dynamical system p X, T q , we have lim

_NÑ8

p 1 { log N q ř

nďN 1

n

f p T

ⁿ

x q µ p n q “ 0 for each f P C p X q and x P X.

(b) By (a) and Tao’s result (“logarithmic Sarnak implies logarithmic Chowla”) [17], in the space M p X

_µ

q of measures on X

_µ

, we obtain that p 1 { log N q ř

nďN

p 1 { n q δ

_Sⁿ_µ

Ñ p

ν

S

, where we consider the Möbius subshift p X

_µ

, S q and p ν

S

stands for the relatively independent extension of the Mirsky measure ν

S

of the square-free system p X

_µ2

, S q .

⁴

(c) By (b) and Theorem 5.1, we obtain that there exists a subsequence p N

_k

q with δ pt N

k

: k ě 1 uq “ 1 such that p 1 { N

k

q ř

nďNk

δ

Sⁿµ

Ñ p ν

S

.

(d) By (c) and the proof of the implication “Chowla implies Sarnak” in [2], it follows that for each zero entropy p X, T q , we have lim

_kÑ8

p 1 { N

_k

q ř

nďNk

f p T

ⁿ

x q µ p n q “ 0 for all f P C p X q and x P X, so Proposition 1.3 follows.

2. Functional formulation of Tao’s result

Our aim in this section is to prove a slight extension of Tao’s result from [18]:

4The measure-theoretic investigations of the square-free systempXµ2, νS, Sqhave been originated by Sarnak [14] and Cellarosi and Sinai [6]: the Mirsky measure is ergodic and so is its relatively independent extension.

Theorem 2.1. Let p B

_j

, } ¨ }

j

q , j “ 1, 2, be normed vector spaces and assume that B

₁

is separable. Let p S

_k

q

kě1

be a sequence of linear bounded operators from B

₁

to B

₂

, such that for some M we have, for each f P B

₁

and each k ě 1,

(11) } S

_k

f }

2

ď M } f }

1

.

⁵

Let φ be a continuous, positive, strictly increasing and convex function on r 0, 8q with φ p 0 q “ 0. Suppose that there exists a subsequence p N

_s

q Ă N such that for any f P B

₁

, setting

(12) R

_f

p H q : “ lim sup

sÑ8

1 log N

_s

ÿ

1ďnďNs

1 n φ

˜›› › › › 1 H

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

¸ ,

we have

(13) lim

HÑ8

R

_f

p H q “ 0.

Then there exists a set N of natural numbers with the property

(14) lim

sÑ8

1 log N

_s

ÿ

NQNďNs

1 N “ 1, such that, for any f P B

₁

, we have

(15) lim

NÑ8, NPN

› ›

› ›

› 1 N

ÿ

1ďnďN

S

_n

f

› ›

› ›

›

2

“ 0.

Moreover, if p N

_s

q “ N , then δ p N q “ 1 and

(16) lim

NÑ8

› ›

› ›

› 1 log N

ÿ

1ďnďN

S

n

f n

› ›

› ›

›

2

“ 0.

The proof of the above theorem requires a few lemmas.

Lemma 2.2. Let G : N Ñ R

_`

. Suppose that for some γ P p 0, 1 q and for some subsequence p N

_s

q Ă N , we have

(17) lim sup

sÑ8

1 log N

s

ÿ

1ďnďNs

G p n q n ď γ.

Then, for the set M Ă N given by M : “ n : G p n q ă ? γ (

, we have

(18) lim inf

sÑ8

1 log N

_s

ÿ

nďNs, nPM

1

n ě 1 ´ ? γ.

Proof. Let Q “ N z M , so that Q “ n : G p n q ě ? γ (

. By Markov’s inequality, we obtain

1 log N

s

ÿ

1ďnďNs

G p n q n ě

? γ log N

s

ÿ

nPQ, nďNs

1 n “

? γ log N

_s

ÿ

nďNs

1 n ´

? γ log N

_s

ÿ

nPM, nďNs

1 n ,

5Assuming thatB₁ is Banach and using the Uniform Boundedness Principle, we only need to assume that in (11) we havesup_kPN}Skf}2ă `8.

hence

1 log N

_s

ÿ

nPM, nďNs

1

n ě 1 log N

_s

ÿ

nďNs

1

n ´ 1

? γ log N

_s

ÿ

1ďnďNs

G p n q n , and (18) holds in view of (17).

Lemma 2.3. Assume that F : N Ñ R

_`

is bounded and satisfies

(19) lim sup

sÑ8

1 log N

s

ÿ

1ďnďNs

F p n q

n ď γ, γ P p 0, 1 q , for a subsequence p N

_s

q . Then, for the set T Ă N given by

(20) T : “

# N : 1

N ÿ

1ďnďN

F p n q ď ? γ +

,

we have

(21) lim inf

sÑ8

1 log N

_s

ÿ

nPT,nďNs

1

n ě 1 ´ ? γ.

Proof. Set G p n q : “

_n¹

ř

1ďmďn

F p m q . By (9), we have ÿ

1ďnďN´1

G p n q

n ` 1 “ ÿ

1ďnďN

F p n q n ´ 1

N ÿ

1ďnďN

F p n q , so that, by (19), we have

lim sup

sÑ8

1 log N

s

ÿ

1ďnďNs

G p n q

n “ lim sup

sÑ8

1 log N

s

ÿ

1ďnďNs´1

G p n q n ` 1 ď γ.

Then, by Lemma 2.2, we obtain (21) for the set T defined by (20).

Lemma 2.4 (see Tao [18]). Assume that M

_k

Ă N , k P N , and that there exists an increasing sequence p N

_s

q Ă N such that, for each k,

sÑ8

lim 1 log N

_s

ÿ

nPMk, nďNs

1

n “ 1, k P N . Then there exists a subset M Ă N such that

(22) lim

sÑ8

1 log N

_s

ÿ

nPM, nďNs

1 n “ 1, and such that, for any k P N , there exists s

_k

with

(23) M X t n : n ě N

_sk

u Ă M

_k

.

Proof. Replacing, if necessary, each M

k

by M

k

X Ş

k¹ăk

M

_k¹

, we may assume without loss of generality that M

_k`1

Ă M

_k

, k P N .

Let us choose an increasing sequence p s

_k

q such that, for each k, s ě s

_k

ñ 1

log N

s

ÿ

nPMk, nďNs

1

n ě 1 ´ 1 k . We set M : “ M

₁

X r 1, N

_s1

s Y Ť

8

k“2

` M

_k

X `

N

_sk´1

, N

_sk

‰˘

and verify that M satisfies the desired properties (22) and (23).

The following is classical.

Lemma 2.5. Let p a

_n

q

ně1

be a bounded sequence in a normed vector space p B , } ¨ }q . Suppose that there exists a subsequence N Ă N , δ p N q “ 1, such that

(24) lim

NÑ8,NPN

› ›

› ›

› 1 N

ÿ

1ďnďN

a

_n

› ›

› ›

› “ 0.

Then

(25) lim

NÑ8

› ›

› ›

› 1 log N

ÿ

1ďnďN

a

_n

n

› ›

› ›

› “ 0.

Proof. Let M “ N z N , so that

(26) lim

NÑ8

1 log N

ÿ

nďN, nPM

1 n “ 0.

By (9), we have ÿ

1ďnďN

a

_n

n “ E

_N

` ÿ

1ďnďN´1

E

_n

n ` 1 , where E

_n

: “ 1 n

ÿ

1ďmďn

a

_m

. Setting C : “ sup

_n

} E

_n

} , we obtain

› ›

› ›

› 1 log N

ÿ

1ďnďN

a

_n

n

› ›

› ›

› ď C log N `

› ›

› ›

› 1 log N

ÿ

nďN, nPM

E

_n

n ` 1

› ›

› ›

› `

› ›

› ›

› 1 log N

ÿ

nďN, nPN

E

_n

n ` 1

› ›

› ›

›

ď C

log N ` C log N

ÿ

nďN, nPM

1 n `

› ›

› ›

› 1 log N

ÿ

nďN, nPN

E

_n

n ` 1

› ›

› ›

› , and then assertion (25) follows from (26) and (24).

Proof of Theorem 2.1. Let f P B

₁

. By (12), (13) and Lemma 2.3, we obtain that for any fixed large H P N , there exists a set N

_f,H

with the property

lim inf

sÑ8

1 log N

_s

ÿ

N_f,HQNďNs

1

N ě 1 ´ b

R

_f

p H q ,

and such that, for N P N

_f,H

, we have 1

N ÿ

1ďnďN

φ

˜›› › › › 1 H

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

¸ ď b

R

_f

p H q .

By deleting at most finitely many elements, we may assume that N

_f,H

consists only of elements of size at least H

²

. For any H

₀

, if we set N

_f,ěH

0

: “ Ť

HěH0

N

_f,H

, then N

_f,ěH

0

satisfies lim

_sÑ8

p 1 { log N

_s

q ř

N_f,ěH

0QNďNs

1

N

“ 1. By Lemma 2.4, we can find a set N

_f

of natural numbers with

sÑ8

lim 1 log N

s

ÿ

N_fQNďNs

1 N “ 1,

and such that, for every H

₀

, every sufficiently large element of N

_f

lies in N

_f,ěH0

. Thus, for every sufficiently large N P N

_f

, one has

1 N

ÿ

1ďnďN

φ

˜›› › › › 1 H

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

¸ ď b

R

_f

p H q ,

for some H ě H

₀

with N ě H

²

. By the monotonicity of φ and Jensen’s inequality, this implies that

φ

˜ 1 N H

› ›

› ›

› ÿ

1ďnďN

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

¸ ď φ

˜ 1 N

ÿ

1ďnďN

› ›

› ›

› 1 H

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

¸

ď 1 N

ÿ

1ďnďN

φ

˜›› › › › 1 H

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

¸ ď b

R

_f

p H q , so that, setting ψ p s q : “ φ

^´1

p ? s q , s ą 0, we get

(27) 1

N H

› ›

› ›

› ÿ

1ďnďN

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

ď ψ p R

_f

p H qq Ñ 0, H Ñ 8 .

Next, by computation, we get ÿ

1ďnďN

ÿ

1ďhďH

S

_n`h

f “ ÿ

1ďhďH

ÿ

1ďnďN

S

_n`h

f “ ÿ

1ďhďH h`N

ÿ

m“h`1

S

_m

f “

H ÿ

N m“1

S

_m

f ` ÿ

1ďhďH N

ÿ

`h m“N`1

S

_m

f ´ ÿ

1ďhďH

ÿ

1ďmďh

S

_m

f,

which gives 1

N ÿ

N m“1

S

_m

f “ 1 HN

ÿ

1ďnďN

ÿ

1ďhďH

S

_n`h

´ 1 HN

ÿ

1ďhďH N`h

ÿ

m“N`1

S

_m

f ` 1 HN

ÿ

1ďhďH

ÿ

1ďmďh

S

_m

f.

Now, let us fix H

₀

. For any sufficiently large N P N

_f

, there exists H ě H

₀

, with H

²

ď N , such that N P N

_f,H

. The triangular inequality yields

1 N

› ›

› ›

› ÿ

N m“1

S

_m

f

› ›

› ›

›

2

ď 1 HN

› ›

› ›

› ÿ

1ďnďN

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

` 1 HN

› ›

› ›

› ÿ

1ďhďH N`h

ÿ

m“N`1

S

_m

f

› ›

› ›

›

2

` 1 HN

› ›

› ›

› ÿ

1ďhďH

ÿ

1ďmďh

S

_m

f

› ›

› ›

›

2

. Using (27), we can upper bound the first term in the right-hand side by ψ p R

_f

p H qq . The sum of the other two terms can be upper bounded by

M } f }

¹

HN

˜ ÿ

1ďhďH N`h

ÿ

m“N`1

1 ` ÿ

1ďhďH

ÿ

1ďmďh

1

¸

ď 2HM } f }

¹

N ď 2M } f }

¹

H . We thus get that

1 N

› ›

› ›

› ÿ

N m“1

S

_m

f

› ›

› ›

›

2

ď sup

HěH0

ψ p R

_f

p H qq ` 2M } f }

1

H

₀

. By letting H

₀

go to infinity, we conclude that lim

_{NÑ8, NP}N_f

› ›

›p 1 { N q ř

N

m“1

S

_m

f › › ›

2

“ 0.

Let now p f

_k

q

kě1

be a dense set in B

₁

. Then, by Lemma 2.4, we get a set N of natural numbers with the property lim

_sÑ8

p 1 { log N

s

q ř

NQNďNs

1

N

“ 1, such that for any k P N ,

NÑ8, NP

lim

N

1 N

› ›

› ›

› ÿ

N m“1

S

_m

f

_k

› ›

› ›

›

2

“ 0.

Take g P B

₁

. Then for any ǫ ą 0 there exists f

_k

such that } g ´ f

_k

}

1

ď ǫ { M, and then 1

N

› ›

› ›

› ÿ

N m“1

S

_m

g

› ›

› ›

›

2

ď 1 N

› ›

› ›

› ÿ

N m“1

S

_m

f

_k

› ›

› ›

›

2

` ǫ, which, by the above, yields

NÑ8, NP

lim

N

1 N

› ›

› ›

› ÿ

N m“1

S

_m

g

› ›

› ›

›

2

“ 0.

So, statement (15) is proved. Assertion (16) follows from (15) by Lemma 2.5.

Note that in the sequel we will use Theorem 2.1 mainly for φ p s q “ s (or sometimes for φ p s q “ s

²

).

3. Proof of Theorem 1.1 To prove Theorem 1.1 we will show a stronger result:

Theorem 3.1. Let p B

_j

, } ¨ }

j

q , j “ 1, 2, be normed vector spaces and assume that B

₁

is separable. Let p S

_k

q

kě1

be a sequence of linear bounded operators from B

₁

to B

₂

, such that for some M ą 0 we have, for each f P B

₁

and each k ě 1,

} S

_k

f }

2

ď M } f }

1

.

Assume that p S

_k

q

kě1

satisfies the property: for all increasing sequences p b

_k

q Ă N with b

_k`1

´ b

k

Ñ 8 and f P B

₁

, we have

KÑ8

lim 1 log b

_K`1

ÿ

kďK

› ›

› ›

› › ÿ

bkďnăb_k`1

1 n S

_n

f

› ›

› ›

› ›

2

“ 0,

Then there exists A Ă N with full logarithmic density: δ p A q “ 1, such that for each f P B

₁

,

AQN

lim

Ñ8

› ›

› ›

› 1 N

ÿ

nďN

S

n

f

› ›

› ›

›

2

“ 0.

The proof of Theorem 3.1 will use the following intermediate result.

Proposition 3.2. Let p B

_j

, } ¨ }

^j

q , j “ 1, 2 be normed vector spaces. Let p S

n

q

ně1

be a sequence of linear bounded operators from B

₁

to B

₂

, such that for any p b

_k

q Ă N with 0 ă b

_k`1

´ b

_k

Ñ 8 as k Ñ 8 , and any f P B

₁

we have

(28) 1

log b

_K`1

ÿ

kďK

› ›

› ›

› › ÿ

b_kďnăbk`1

1 n S

_n

f

› ›

› ›

› ›

2

ÝÝÝÝÑ

KÑ8

0.

Then

(29) lim

_HÑ8

lim

_NÑ8

1

log N ÿ

nďN

1 n

› ›

› ›

› 1 H

ÿ

hďH

S

_n`h

f

› ›

› ›

›

2

“ 0.

We will prove this result by contraposition, and we will need the following lemma.

Lemma 3.3 (Diagonalization Lemma). Consider a family of sequences p g

_n,m

q Ă R

_`

, m ą n, m, n ě 1. Suppose that for some families p b

_k,ℓ

q

k,ℓě1

Ă N with

(30) 0 ă b

_k,ℓ

ă b

_k`1,ℓ

, lim

_ℓÑ8

lim

_kÑ8

p b

_k`1,ℓ

´ b

_k,ℓ

q “ 8 , and some γ ą 0, we have

(31) lim

_KÑ8

1

log b

_K`1,ℓ

ÿ

K k“1

g

_{bk,ℓ,bk`1,ℓ}

ě γ, for all ℓ P N . Then there exists a sequence p b

_k

q

kě1

Ă N such that

(32) 0 ă b

_k

ă b

_k`1

, b

_k`1

´ b

_k

ÝÝÝÑ

kÑ8

8 , and

(33) lim

_KÑ8

1

log b

_K`1

ÿ

K k“1

g

_bk,bk`1

ě γ { 2.

Proof. Note that by (30), we have b

_k`1,1

´ b

_k,1

ě 1 for each k ě 1, and that without loss of generality we may assume that for each ℓ ě 1, we have lim

_kÑ8

p b

_k`1,ℓ

´ b

k,ℓ

q ě ℓ. By (31) for ℓ “ 1, we can choose the value K

₁

so that

1 log b

K1`1,1

K1

ÿ

k“1

g

b_k,1,b_k`1,1

ě γ { 2 and take then, for k “ 1, . . . , K

₁

` 1, b

_k

: “ b

_k,1

, to obtain

1 log b

_K1`1

K1

ÿ

k“1

g

_bk,bk`1

ě γ { 2.

We continue the above process by induction. Suppose that for some ℓ ě 1 we already have sequences 0 “ K

₀

ă K

₁

ă ¨ ¨ ¨ ă K

_ℓ

, 1 ď b

₁

ă b

₂

ă ¨ ¨ ¨ ă b

_Kℓ`1

, satisfying, for each s “ 1, . . . , ℓ,

(34) b

_k`1

´ b

_k

ě s, k “ K

_s´1

` 1, . . . , K

s

, and

(35) 1

log b

_Ks`1

Ks

ÿ

k“1

g

b_k,b_k`1

ě γ { 2.

Then, at step ℓ ` 1, we first choose N

_ℓ`1

large enough so that b

_{Kℓ`2`Nℓ`1,ℓ`1}

ě b

_Kℓ`1

` ℓ ` 1, and, for each k ě K

_ℓ

` 2 ` N

_ℓ`1

, b

_k`1,ℓ`1

´ b

_k,ℓ`1

ě ℓ ` 1. Then, by (31), we can choose K

_ℓ`1

ą K

ℓ

` 2 large enough so that

1

log b

_{Kℓ`1`1`Nℓ`1,ℓ`1}

Kℓ`1

ÿ

`Nℓ`1

k“Kℓ`2`Nℓ`1

g

_{bk,ℓ`1,bk`1,ℓ`1}

ě γ { 2,

and we take for k “ K

_ℓ

` 2, . . . , K

_ℓ`1

` 1 :

b

_k

: “ b

_{k`Nℓ`1,ℓ`1}

.

Then assertions (34) and (35) are valid up to s “ ℓ ` 1, and this allows us to construct

inductively the sequences p b

k

q

kě1

with the required properties (32), (33).

Proof of Proposition 3.2. Suppose that for some f P B

₁

, (29) does not hold. Then, there exist γ ą 0 and a sequence p H

_ℓ

q , H

_ℓ

Ñ 8 as ℓ Ñ 8 , such that for each ℓ ě 1, we have

lim

_NÑ8

1 log N

ÿ

1ďnďN

1 n

› ›

› ›

› 1 H

_ℓ

ÿ

1ďhďHℓ

S

_n`h

f

› ›

› ›

›

2

ě γ.

We write this in the form lim

_NÑ8

1

log N 1 H

_ℓ

H

ÿ

ℓ´1 r“0

ÿ

nďN, n”rrHℓs

1 n

› ›

› ›

› ÿ

1ďhďHℓ

S

_n`h

f

› ›

› ›

›

2

ě γ.

Then there exists r

_ℓ

, 0 ď r

_ℓ

ă H

_ℓ

, such that lim

_NÑ8

1

log N

ÿ

nďN, n”rℓrHℓs

1 n

› ›

› ›

› ÿ

1ďhďHℓ

S

_n`h

f

› ›

› ›

›

2

ě γ.

Using 1 n “ 1

n ` h ` h

n p n ` h q , lim

NÑ8

1 log N

ÿ

nďN, n”rℓrHℓs

H

_ℓ²

n

²

“ 0, we obtain

lim

_NÑ8

1 log N

ÿ

nďN, n”rℓrHℓs

› ›

› ›

› ÿ

1ďhďHℓ

1

n ` h S

_n`h

f

› ›

› ›

›

2

ě γ, or

lim

_NÑ8

1 log N

ÿ

nďN, n”rℓrHℓs

› ›

› ›

› ÿ

nămďn`Hℓ

1 m S

m

f

› ›

› ›

›

2

ě γ.

Rewrite this inequality in the form lim

_NÑ8

1

log N

rN

ÿ

{Hℓs k“1

› ›

› ›

› ›

ÿ

kH_ℓ`rℓămďpk`1qHℓ`rℓ

1 m S

_m

f

› ›

› ›

› ›

2

ě γ,

and take, for a fixed ℓ, the sequence p b

_k,ℓ

q

kě1

defined by b

_k,ℓ

: “ kH

_ℓ

` r

_ℓ

` 1. Setting K

_N

: “ r N { H

_ℓ

s for each N, we have log b

_KN`1,ℓ

{ log N ÝÝÝÝÑ

NÑ8

1, hence

lim sup

KÑ8

1 log b

_K`1,ℓ

ÿ

kďK

› ›

› ›

› ›

ÿ

bk,ℓďmăb_k`1,ℓ

1 m S

m

f

› ›

› ›

› ›

₂

ě γ.

We can now apply the Diagonalization Lemma 3.3 with the sequences g

_n,m

: “

› ›

› ›

› ÿ

nďjăm

1 j S

_j

f

› ›

› ›

›

2

.

We obtain that there exists a sequence p b

_k

q

kě1

with 0 ă b

_k`1

´ b

_k

ÝÝÝÑ

kÑ8

8 , such that lim

_KÑ8

1

log b

_K`1

ÿ

kďK

› ›

› ›

› › ÿ

b_kďmăbk`1

1 m S

m

f

› ›

› ›

› ›

2

ě γ { 2.

Hence (28) is not satisfied

Proof of Theorem 3.1. By Proposition 3.2, we obtain that if we set R

_f

p H q : “ lim sup

NÑ8

1 log N

ÿ

1ďnďN

1 n

› ›

› ›

› 1 H

ÿ

1ďhďH

S

_n`h

f

› ›

› ›

›

2

,

then lim

_HÑ8

R

_f

p H q “ 0, so the result follows from Theorem 2.1 with φ p s q “ s.

Proof of Theorem 1.1. We apply Theorem 3.1 to B

₁

“ B

₂

“ C p X q and S

_k

p f q : “ µ p k q f ˝ T

^k

.

4. Proof of Corollary 1.2 and related results

Recall that a point y in a topological dynamical system p Y, S q is quasi-generic for some measure ν if, for some subsequence p N

_k

q of integers and all f P C p Y q , we have

1 N

k

ÿ

1ďnďNk

f p S

ⁿ

y q ÝÝÝÑ

kÑ8

ż

Y

f dν.

Likewise, we say that y is logarithmically quasi-generic for ν if, for some subsequence p N

_k

q of integers and for all f P C p Y q , we have

1 log N

_k

ÿ

1ďnďNk

1

n f p S

ⁿ

y q ÝÝÝÑ

kÑ8

ż

Y

f dν.

Observe that any measure for which y is logarithmically quasi-generic is S-invariant.

We will use here the following result from [10] (see the remark after Theorem 1.3 therein).

Theorem 4.1 (Frantzikinakis and Host). Let p Y, S q be a topological dynamical sys- tem, and let y P Y . Assume that, for any measure ν for which y is logarithmically quasi-generic, the system p Y, ν, S q has zero entropy and countably many ergodic com- ponents. Then for any g P C p Y q , we have

(36) lim

NÑ8

1 log N

ÿ

1ďnďN

1

n g p S

ⁿ

y q µ p n q “ 0.

Proof of Corollary 1.2. Let us consider a dynamical system p X, T q with zero topolog- ical entropy, and such that M

^e

p X, T q is countable. In view of Theorem 1.1, all we need to prove is that p X, T q satisfies the logarithmic strong MOMO property. That is, we fix an increasing sequence p b

_k

q Ă N with b

_k`1

´ b

_k

Ñ 8 (and we assume without loss of generality that b

₁

“ 1), a sequence p x

_k

q Ă X and f P C p X q , and we have to show the following convergence

(37) 1

log b

_K`1

ÿ

kďK

ˇ ˇ ˇ ˇ ˇ ˇ

ÿ

bkďnăb_k`1

1

n f p T

ⁿ

x

_k

q µ p n q ˇ ˇ ˇ ˇ ˇ ˇ ÝÝÝÝÑ

KÑ8

0.

According to [3, Lemma 18], it is sufficient to show that

(38) 1

log b

_K`1

ÿ

kďK

e

_k

ÿ

b_kďnăbk`1

1

n f p T

ⁿ

x

_k

q µ p n q ÝÝÝÝÑ

KÑ8

0, where e

_k

P Σ

₃

: “ t e

^2πij{3

: j “ 0, 1, 2 u is chosen so that the product

e

_k

ÿ

bkďnăb_k`1

1

n f p T

ⁿ

x

_k

q µ p n q

belongs to the closed cone t 0 u Y t z P C : arg p z q P r´ π { 3, π { 3 su .

In order to show (38), we consider the space Y : “ p X ˆ Σ

₃

q

^N

with the shift S, and in this system the point y “ p y

_n

q

nPN

defined by

y

_n

: “ p T

ⁿ

x

_k

, e

_k

q if b

_k

ď n ă b

_k`1

p k ě 1 q .

Let ν be a measure for which y is logarithmically quasi-generic. The same argument as in [3] (see the proof of (P2 ñ P3)) shows that ν must be concentrated on the set of sequences of the form

` p x, a q , p T x, a q , p T

²

x, a q , . . . ˘

p x P X, a P Σ

₃

q .

Now, let us consider an ergodic component ρ of ν. The marginal of ρ on the first coordinate x must be an ergodic T -invariant measure on X. By assumption, there are only countably many of them, and all of them give rise to zero-entropy systems. The marginal of ρ on the second coordinate a is one of the three Dirac measures δ

₁

, δ

_ei2π{3

, δ

_ei4π{3

. Moreover these two marginals must be independent by the disjointness of ergodic systems with the identity, thus, these two marginals completely determine ρ.

Hence, we see that there can be only countably many possible ergodic components of ν, and all of them have zero entropy. Thus y satisfies the assumptions of Theorem 4.1, and we have (36) for each g P C p Y q . In particular, if we take the continuous function g defined by g p z q : “ a

₀

f p z

₀

q for each z “ pp z

₀

, a

₀

q , p z

₁

, a

₁

q , . . . q P Y , we obtain (38).

Remark 4.2. We can characterize uniform convergence for Möbius orthogonality in terms of a MOMO type convergence. Indeed:

The uniform convergence in Möbius orthogonality (1) holds if and only if for all p b

_k

q satisfying b

_k

{ b

_k`1

Ñ 0, we have

1 b

_k`1

› ›

› ›

› › ÿ

b_kďnăbk`1

µ p n q f ˝ T

ⁿ

› ›

› ›

› ›

CpXq

ÝÝÝÑ

kÑ8

0.

To see this equivalence, it is sufficient to note that for each x P X, 1

b

_k`1

ÿ

bkďnăb_k`1

f p T

ⁿ

x q µ p n q “ 1 b

_k`1

ÿ

1ďnăb_k`1

f p T

ⁿ

x q µ p n q

´ b

_k

b

_k`1

1 b

_k

ÿ

1ďnăbk

f p T

ⁿ

x q µ p n q .

Remark also that the same arguments work for the logarithmic averages (replacing b

_k

{ b

_k`1

Ñ 0 with log b

_k

{ log b

_k`1

Ñ 0).

5. Miscellanea

5.1. Ergodic measures. In this section, we show that Tao’s approach persists, if we consider the main observation from [12].

Let p X, T q be a dynamical system. Given x P X and n P N , we write δ

_Tⁿ_pxq

for the Dirac measure concentrated at the point T

ⁿ

p x q . Let

E p x, N q : “ 1 N

ÿ

1ďnďN

δ

_Tⁿ_pxq

, E

^log

p x, N q : “ 1 log N

ÿ

1ďnďN

1

n δ

_Tⁿ_pxq

.

We consider here the convergence of these empirical measures in the weak* topology.

Note that any accumulation point of the above sequences is always a T -invariant probability measure on p X, T q .

Theorem 5.1. Suppose that for some x P X and some subsequence p N

s

q

sě1

of natural numbers, we have

(39) lim

sÑ8

E

^log

p x, N

_s

q “ κ, where κ is ergodic.

Then there exists a set N of natural numbers with the property

(40) lim

sÑ8

1 log N

s

ÿ

NQNďNs

1 N “ 1, such that

(41) lim

NÑ8, NPN

E p x, N q “ κ.

Proof. The condition (39) means that for any f P C p X q we have

(42) lim

sÑ8

1 log N

_s

Ns

ÿ

n“1

1

n f p T

ⁿ

u q “ ż

X

f dκ.

Let f P C p X q be fixed, and set Sf : “ ş

X

f dκ. For H P N , we consider the limiting of the second moment

R

_f

p H q : “ lim

sÑ8

1 log N

_s

ÿ

1ďnďNs

1 n ˇ ˇ ˇ ˇ ˇ

1 H

ÿ

H m“1

f p T

^n`m

x q ´ Sf ˇ ˇ ˇ ˇ ˇ

2

.

The limit does exist by (39), as the internal is given by a continuous function sum sampled at x. So, by condition (42), we have

(43) R

f

p H q “

ż

X

ˇ ˇ ˇ ˇ ˇ

1 H

ÿ

H m“1

f p T

^m

x q ´ Sf ˇ ˇ ˇ ˇ ˇ

2

dκ p x q .

Hence, directly by the von Neumann ergodic theorem, and using the ergodicity of κ, we obtain lim

_HÑ8

R

_f

p H q “ 0. Take now B

₁

“ C p X q , B

₂

“ C with the sequence of functionals S

k

f : “ f p T

^k

x q ´ Sf , k P N , and we obtain statement (41) by Theorem 2.1.

5.2. Davenport-Erdös theorem. Davenport-Erdös theorem [7] is the fact that, given B Ă N , the B -free set F

_B

, i.e. the set of those numbers that have no divisor in B , has logarithmic density and, moreover, δ p F

_B

q “ d p F

_B

q . In fact, see [8], the point

¹F_B

is logarithmically generic for the relevant Mirsky measure which is ergodic. Hence, by Theorem 5.1, we obtain that the upper asymptotic density d p F

_B

q is obtained along a subsequence of logarithmic density 1. We can however obtain this result in an elementary way. Indeed, for a subset A Ă N and N P N , set d

_N

p A q : “ 1

N ř

1ďnďN¹A

p n q and d

^log_N

p A q : “ 1 log N

ř

1ďnďN 1

n¹A

p n q . More generally, given a “ p a

n

q

nPN

a sequence of real numbers, set d

N

p a q : “ 1

N ř

1ďnďN

a

n

and d

^log_N

p a q : “ 1

log N ř

1ďnďN an

n

.