getdocd947. 207KB Jun 04 2011 12:05:03 AM

El e c t ro n ic

Jo ur

n a l o

f P

r o

b a b i_{l i t y}

Vol. 13 (2008), Paper no. 63, pages 1927–1951. Journal URL

http://www.math.washington.edu/~ejpecp/

A special set of exceptional times for dynamical random

walk on

Z

2

Gideon Amir

∗

Christopher Hoffman

†

Abstract

Benjamini, Häggström, Peres and Steif introduced the model of dynamical random walk onZd

[2]. This is a continuum of random walks indexed by a parameter t. They proved that for d=3, 4 there almost surely existtsuch that the random walk at timetvisits the origin infinitely often, but ford≥5 there almost surely do not exist such t.

Hoffman showed that ford=2 there almost surely existstsuch that the random walk at timet visits the origin only finitely many times[5]. We refine the results of[5]for dynamical random walk onZ2_{, showing that with probability one the are times when the origin is visited only a}

finite number of times while other points are visited infinitely often.

Key words:Random Walks; Dynamical Random Walks, Dynamical Sensitivity. AMS 2000 Subject Classification:Primary 60G50 ; 82C41.

Submitted to EJP on October 9, 2006, final version accepted October 20, 2008.

1

Introduction

We consider a dynamical simple random walk on Z2. Associated with each nis a Poisson clock. When the clock rings the nth move of the random walk is replaced by an independent random variable with the same distribution. Thus for any fixed t the distribution of the walks at time t is that of simple random walk onZ2and is almost surely recurrent.

More formally let_{Y_nm}m,n∈N be uniformly distributed i.i.d. random variables chosen from the set {(0, 1),(0,−1),(1, 0),(₋1, 0)_}.

For each n, let{τ(m)

n }m≥0 be the arrival times of a Poisson process of rate 1, with the processes for

differentnindependent of one another andτ(0)

n =0 for eachn.

Define

X_n(t) =Y_nm

for allt_∈[τ(_nm),τ(_nm+1)). Let

S_n(t) =

n

X

i=1

X_i(t).

Thus for eacht the random variables{X_n(t)_}n∈N are i.i.d.

The concept of dynamical random walk (and related dynamical models) was introduced by Ben-jamini, Häggström, Peres and Steif in [2], where they showed various properties of the simple random walk to hold for all times almost surely (such properties are called “dynamically stable"), while for other properties there a.s. exist exceptional times for which they do not hold (and are then called “dynamically sensitive" properties). In particular, they showed that transience of simple random walk is dynamically sensitive in 3 and 4 dimensions, while in dimension 5 or higher it is dynamically stable. For more work on dynamical models see also[8]and[4].

In[5]Hoffman showed that recurrence of simple random walk in two dimensions is dynamically sensitive, and that the Hausdorff dimension of the set of exceptional times is a.s. 1. Furthermore it was showed that there exist exceptional times where the walk drifts to infinity with rate close to

p

n.

The following problem was raised by Jeff Steif and Noam Berger during their stay at MSRI: When the dynamical random walk on Z2 _{is in an exceptional time, returning to the origin only a finite}

number of times, is it always because it “escapes to infinity”, or are there times where the disc of radius 1 is hit infinitely often, but the origin is hit only a finite number of times.

In this paper we prove that such exceptional times exist, and furthermore that the Hausdorff di-mension of the sets of these exceptional times is 1. The techniques used are a refinement of the techniques used in[5]to show the existence of exceptional times where the walk is transient.

The proof is divided into two main parts. First, we define a sequence of eventsS E_k(t), dependent on

{Sn(t)}n∈N such that T

S Ek(t)implies we are in an exceptional time of the desired form, and show

that the correlation between these events in two different times t1,t2 is sufficiently small (Theorem

2

Definitions, Notation and Main Results

We will follow the notation of[5]as much as possible. LetDdenote the discrete circle of radius 1:

D₌ n

(1, 0),(₋1, 0),(0, 1),(0,₋1) o

.

The set of special exceptional times of the process, Sexc, is the set of all times for which the walk visitsD_{infinitely often but visits}(0, 0)only fintely often. More explicitly we define

Sexc= ½

t:

¯ ¯

¯{n:Sn(t)∈D} ¯ ¯

¯=∞ and ¯ ¯

¯{n:Sn(t) =0} ¯ ¯ ¯<∞

¾

.

The main result of this paper is:

Theorem 1. P(Sexc₆=_;) =1. Furthermore the Hausdorff dimension of Sexcis1a.s.

We will need the following notation. Define a series of times

sk=k1022k

2

We will divide the walk into segments between two consecutivesi:

M_j=_{i∈N_:sj−1≤i<sj},

And define a super segment to be a union of segments

W_k= [

2k_≤j<2k+1

M_j=_{i_∈N_{: s2}k₋₁≤i<s₂k+1}.

Define a series of annuli

Ak=

n

x∈Z2 : 2k2_{≤ |}x| ≤k1022k2o.

where_|x_|denotes the standard Euclidean norm_|x_|=px₁2+x₂2.

And define eventG_kby

G_k(t) =_{S_s

k(t)∈Ak}.

Choosing these annuli to be of the right magnitude will prove to be one of the most useful tools at our disposal. We will show that a random walk will typically satisfyGk (see Lemmas 6 and 7). On

the other hand, the location of the walk inside the annuli A_j in the beginning of the segment M_j will have only a small influence of the probabilities of the events we will investigate (see Lemmas 8 and 10). Together this provides “almost independence" of events in different segments.

We will define three events concerned with hitting the disc on the dynamical random walk. We define the event R_j(t), to be the event that the walk (at time t) hits D _{at some step in in the}

(Note that in[5], this denoted the event of hitting the origin, and not D_{, but this difference does}

not alter any of the calculations.)

We denote bySRj(t)the event that the walk (at timet) hitsDat some step in the segmentMj, but

This is the event that the walk never hits the origin in the super segmentW_k, and there is exactly one segmentM_j, 2k_≤ j<2k, where the walk hitsD_{. Additionally this event requires the walk ends}

We will use the following notation for conditional probabilities. Let

Px,k−1(_∗) =P€∗|Ssk−1(0) =x

To prove Theorem 1 we will first prove the following theorem.

The first statement in Theorem 1 follows from Theorem 2 and the second moment method. The second statement in Theorem 1 will follow from bounds that we will obtain on the growth of f(t)

near zero and by Lemma 5.1 of[11].

Since we will be showing that some events have low correlation between times 0 and t, some of the bounds we use to prove Theorem 2 will only hold whenk is sufficiently large compared to 1_t. Therefore, fort_∈(0, 1), we denote byK(t)the smallest integer greater than_|logt_|+1, and byK′(t)

the smallest integer greater than log(_|logt_|+1)+1. (Here and everywhere we use log(n) =log₂(n)).

We end this section by stating that we use C as a generic constant whose value may increase from line to line and lemma to lemma. In many of the proofs in the next section we use bounds that only hold for sufficiently largek(or j). This causes no problem since it will be clear that we can always chooseC such that the lemma is true for allk.

3

Proof of Theorem 2

This section is divided into four parts. In the first, we introduce some bounds on the behavior of two dimensional simple random walk. These are quite standard, and are brought here without proof. In the second part we quote, for the sake of completeness, lemmas from[5]which we will use later on. In the third part, which constitutes the bulk of this paper, we prove estimates on

P(S E_k(0)_| S E_k−1(0))andP(S E_k(0,t)_| S E_k−1(0,t)). In the fourth part we use the these bounds to prove Theorem 2.

3.1

Two Dimensional Simple Random Walk Lemmas

The main tool that we use are bounds on the probability that simple random walk started at x returns to the origin before exiting the ball of radiusnwith center at the origin. For general x, this probability is calculated in Proposition 1.6.7 on page 40 of[7]in a stronger form than given here. The estimate for|x|=1 comes directly from estimates on the 2−d harmonic potential (Spitzer 76, P12.3 page 124 or[6]).

Letηbe the smallestm>0 such thatS_m(0) =0or|S_m(0)_{| ≥}n.

Lemma 3. 1. There exists C>0such that for all x with0<_|x_|<n

log(n)₋log_|x_{| −}C

log(n) ≤P(Sη(0) =0|S0(0) =x)≤

log(n)₋log_|x_|+C log(n) .

2. For_|x_|=1we have the following stronger bound:

P(_|S_η(0)_{| ≥}n_|S₀(0) =x) = π

2 logn+C +O(n

−1_).

We will also use the following standard bounds.

Lemma 4. There exists C >0such that for all x_∈Z2_{, n∈}N_{and m<}p_n

and

P

‚

|S_n(0)₋x|< p_n

m

Œ

≤ C

m2. (2)

Lemma 4 is most frequently applied in the form of the following corollary:

Corollary 5. There exists C >0, such that for any n₀,N,M >0, if_|S_n0(0)_{| ≥}M then

P(_∃n such that n₀<n<n₀+N and _|S_n(0)_{| ≤}1)_≤ C N

M2.

3.2

Some Useful Lemmas From

[

5

]

First we list some lemmas from[5]that we will use in the proof. (In[5]R_j(0)is defined as

R_j(0) =_{∃m_∈M_j such thatS_m(0) =0_}.

It is easy to see all lemmas in[5]hold with our slightly expanded definition.)

Lemma 6 shows that if the walk is in the annulusA_j−1 at steps_j−1, then with high probability the walk will be in the annulus A_j at steps_j. This will allow us to condition our estimates of various events on the event thatSsj ∈Aj.

Lemma 6. For any j and x ∈Aj−1

Px,j−1((Gj(0))C)≤

C

j10.

We will usually use the following corollary of Lemma 6, which is just an application of Lemma 6 to all segments inside a super-segment, and follows directly by applying the union bound:

Corollary 7. For any k>0and x ∈A₂k₋₁

Px,2k−1



 

\

2k_≤l<2k+1

Gl(0)

c 

 ≤

C

29k.

Lemma 8 shows that we have a good estimate of the probability of hittingDduring the segmentMk,

given the walk starts the segment inside the annulusA_k−1.

Lemma 8. There exists C such that for any k and any x∈Ak−1

2 k−

Clogk

k2 ≤P

x,k−1_(R

k(0))≤

2 k +

Clogk

k2 .

Lemma 9. There exists C such that for all k, n_≥s_k−1+s_k/210k, for all I_{⊂ {}1, . . . ,n_}with_|I_{| ≥}s_k/210k and for all{Xi(t)}i∈{1,...,n}\I

P€_∃j∈ {n, . . . ,sk}such that Sj(t)∈D

¯

¯ {x_i(t)}i_{∈{1,...,n}\I})≤

C k.

(Where we interpret the set_{n, . . . ,s_k}as the empty set for n>s_k).

The last lemma we quote says there is a low correlation between hittingD (during some segment Mk) at different times.

Lemma 10. There exists C such that for any t, any k>K(t)and any x,y ∈Ak−1

Px,y,k−1(Rk(0,t))≤

C

k2.

3.3

Estimating P(

S E

_k

(

0

))

and P(

S E

_k

(

0,

t

))

We start by giving estimates for the eventSR_j(t).

Lemma 11. There exists C >0, such that for any j and any x∈Aj−1,

π

j3 −

Clogj j4 ≤P

x,j−1_(SR j(0))≤

π

j3 +

Clogj j4 .

Proof. Lethbe the first step in the segment M_j such thatS_h(0)_∈D_{, lettingh=∞if no such step}

exists. By Lemma 8,

2 j −

Clogj j2 ≤P

x,j−1_(h<∞)≤ 2

j + Clogj

j2 . (3)

LetB_i denote the event that the walk does not hit0between stepiands_j. Then

Px,j−1(SRj(0)∩Gj(0))

= X

sj−1≤i<sj

Px,j−1(h=i)P(Bi∩Gj(0)|h=i). (4)

For anys_j−1≤i<s_j,B_i∩G_j(0)is included in the event that after stepi, the walk reachesA_j before reaching0. Therefore by the second statement in Lemma 3 there existsC such that for alli

P€B_i(0)_∩G_j(0)_|h=iŠ_≤ π 2j2 +

C

j4. (5)

Px,j−1€SR_j(0)_∩G_j(0)Š

Thus using (6) and (7) together with conditional probabilities with respect toG_j(0), we get

Px,j−1(SRj(0)) ≤ Px,j−1

which establishes the upper bound.

To get the lower bound, note that conditioned on the eventGj(0), if after stepithe walk reachesAj

before reaching(0, 0), and then does not hit(0, 0)in the nexts_jsteps, thenB_i occurs.

LettingE1denote the event that after stepiwe reachAjbefore reaching(0, 0), and lettingE2denote

the event that a simple random walk, starting atA_j, does not hit(0, 0)in the nexts_j steps. We get, using the Markov property, that

Px,j−1(B_i|G_j(0)_{∩ {}h=i_})_≥P(E_{1 |}h=i)P(E₂). (8)

And by the Markov property and stationarity, since a walk starting atA_j must hitA_j−1 before

Combining (9) and (10) into (8) we get

Px,j−1(B_i |G_j(0)_{∩ {}h=i}) _≥

_π

2j2 − C

j4 1− 2

j − Clogj

j2

≥ π

2j2 − C

j3. (11)

Using (11) for all jand x_∈A_j−1

Px,j−1(SR_j(0)_|G_j(0)) = X

i

Px,j−1(h=i)P(B_i|G_j(0)_{∩ {}h=i_})

≥ Px,j−1(h<_∞)inf

i P(Bi|Gj(0)∩ {h=i})

≥

₂

j − Clogj

j2

·

_π

2j2 − C

j3

≥ π

j3−

Clogj

j4 . (12)

Thus for all jandx _∈A_j−1, conditioning onG_j(0)and using Lemma 6 and (12) implies that

Px,j−1(SR_j(0)) _≥ Px,j−1(SR_j(0)_|G_j(0))₋Px,j−1(G_j(0)c)

≥ π

j3 −

Clogj j4 −

C j10

≥ π

j3 − Clogj

j4 .

The next two lemmas give a lower bound onP(S Ek(0)). We define

β_k:= Y 2k_≤j<2k+1

1₋2 j

. (13)

This is an approximation of the probability of never hittingD _{during the super segmentWk. Note}

that there existsc>0 such thatc< β_k<1 for allk>1.

Lemma 12. For any k and any2k≤ j<2k+1, and any x∈A₂k₋₁

Px,2k−1S E_kj(0)_≥β_kπ

j3 − Clogj

j4 .

Px,2k−1

Last, by Lemma 6

Putting (15),(16) and (17) into (14) we get

The inequality in (18) follows from Lemma 12.

Next we will work to bound from above the probability ofS Ek(0,t).

Lemma 14. There exists a constant C>0such that for any k>K′(t)and any j₁₆= j₂, 2k_≤ j₁,j₂<

So by the Markov property,

max

Now we will estimate each of the four terms in (19).

max

The last inequality holding by Lemma 11.

Similarly, by symmetry between times 0 andt,

max

To estimate the third term in (19) we use Lemma 10 to estimate each term in the product. Lemma 10 says that there existsC>0 such that for anyl and anyx,y∈Al−1

Px,y,l−1(R_l(0,t))_≤ C

And the lower bound from Lemma 8:

Px,l−1(R_l(0))_≥ 2

l − Clogl

l2 .

We then get that

max

For the fourth factor in (19), Corollary 7 gives

Combining (20), (21), (22) and (23) into (19) we get

it holds with only negligible probability.

Lemma 15. There exists C >0such that for any t>0, k>K′(t),

symmetry, we can estimate

Px,y,j−1 SR_j(0,t)

Combining these we get

Thus we get

Px,y,j−1 SR_j(0,t)

≤ 2Px,y,j−1 SR_j(0,t)_{∩ {}n_0≤n_t}

≤ 2Px,y,j−1 SR_j(0,t)_∩G_j(0,t)_{∩ {}n₀≤nt}

+2Px,y,j−1((G_j(0,t))c)

≤ 2Px,y,j−1 Rj(0)

·Px,y,j−1 L1 |Rj(0)

·Px,j−1 Rj(t)|Rj(0)∩L1

· ·Px,y,j−1 L_2|R_j(0,t)_∩L₁

+ 2Px,y,j−1((G_j(0,t))c). (24)

To bound the probabilities of the five terms on the right hand side of (24) we now introduce six new events, bound the probabilities of these events and then use these bounds to bound the terms in (24).

LetB₁ be the event that

n0<sj−1+

22(j−1)2 j6 .

Recall that by Corollary 5, for anyM,N >0, if_|S_s

j−1(0)|>M, then

P(_∃s_j−1<n′<s_j−1+N : _|S_n′(0)| ≤1)≤

C N M2.

Since for anyx _∈A_j−1we have_|x_{| ≥}2(j−1)2, we can set M=2(j−1)2andN= 22(j−1)

2

j6 to conclude

Px,y,j−1(B₁) = Px,y,j−1 _∃s_j−1<n′<s_j−1+2 2(j−1)2

j12 : |Sn′(0)| ≤1 !

≤

C22(j−1)

2

j6

(2(j−1)2

)2

≤ C

j6. (25)

Let I denote the set of all indices between s_j−1 and s_j−1+ 22(j−1)

2

j6 for which, conditioned on our

Poisson process,Xi(0)andXi(t)are independent.

LetB2 be the event that|I|< 2

2(j−1)2 2j+2_j6 .

IfB₁ does not occur, then_|I_|is the sum of at least 22(j−1)

2

j6 i.i.d. indicator variables each happening

with probability

p=1−et>1

2min(1,t)≥ 1

2j+1,

(the last inequality holds because j_≥2k _≥K(t)_{≥ |}logt_|). Therefore by monotonicity int and in

the number of indicators,_|I|stochastically dominates the sum ofN= 22(j−1)

2

As we are conditioning on(B₁)cwe have that_|I_|dominatesT and ifT _≥ N P₂ then

|I_{| ≥} N P

2 =

22(j−1)2 2j+2_j6

andB₂does not occur.

By the Chernoff bound

P

T< N p

2

≤P

|T₋N p_{| ≥} N p 2

≤2e−cN p

for some absolute constantc>0. Thus

Px,y,j−1 B_{2 |}(B₁)c

≤P

T> N p

2

≤2e−

c22(j−1)2 2j+2j6 _≤ C

j6. (26)

Putr= 2(j−1)

2

2j+2_j6.

LetB3 be the event that|Sn0(t)|<r and

Sn0(t) =

X

i<n0

Xi(t).

Rearranging the order of summation we can write

Sn0(t) =

X

i<n0

Xi(t) =

X

i<n0,i∈/I

Xi(t) +

X

i∈I

Xi(t).

Put

x= X

i<n0,i∈/I

X_i(t).

Since all variables_{X_i}i∈I have been re-rolled between time 0 andt, the probability that_|S_n

0(t)<

r|is the same as the probability that simple random walk starting at x will be at distance less than rfrom(0, 0)after|I|steps. Therefore by the second part of Lemma 4

Px,y,j−1(S_n

0(t)<

p

|I_| j3 )≤

C

j6 (27)

And since(B₂)c implies_|I_|> 22(j−1)

2

2j+2_j6 >r

2_j6 _{we get}

Px,y,j−1(B_{3 |}(B₂)c)< C

j6 (28)

Next we bound the probability that n_t−n₀ is small. LetB₄ be the event that 0<n_t−n₀ < r_j62 =

22(j−1)2

from its position at stepn0 in less than r

2

j6 steps, therefore we can apply the first part of Lemma 4 to

bound

Px,y,j−1(B_4|(B₃)c)< C

j6. (29)

LetI₁denote the set of all indices betweenn₀andn_tfor which, conditioned on our Poisson process,

X_i(0)andX_i(t)are independent. LetB₅ denote the event that_|I_1|< 22(j−1)

2

24(j+2)_j1₂. A similar calculation

as done forB₂ shows thatPx,y,j−1(B₅)< C_j6

LetB6be the event that|Snt(0)|<r1. Using Lemma 4 in a calculation similar to the one forB3gives

that

Px,y,j−1(B6 |(B5)c)<

C

j6. (30)

Thus we get

Px,y,j−1(Bi)≤

C

j6 i=1, . . . , 6. (31)

We will estimate the five probabilities in (24) as follows:

1. By Lemma 8,

Px,y,j−1 R_j(0)

=Px,j−1 R_j(0)

≤ C

j. (32)

2. L₁ is included in the event that after step n₀, at for which _|S_n0(0)_| = 1, the walk reaches distancer before hitting(0, 0), therefore by Lemma 3,

Px,y,j−1 L_{1 |}R_j(0)

≤ C

logr1 ≤

C

j2. (33)

3. To estimatePx,y,j−1 Rj(t)|Rj(0)∩L1

, notice thatRj(0)andL1depend only on what happens

at time 0, and(B₂)cimplies_|I_{| ≥} sj

210j (For all large enough j) , so using Lemma 9 we get

Px,y,j−1 Rj(t)|(B2)c∩Rj(0)∩L1

≤ C

j,

and therefore

Px,y,j−1 Rj(t)|Rj(0)∩L1

≤ C

j + C

j6 ≤ C

j. (34)

4. To estimatePx,y,j−1 L_{2 |}R_j(0,t)_∩L₁

, we partition according to the value ofn_t. Since for a givennt, the event L2is independent of the variables{Xi(0)}i<nt∪ {Xi(t)}i<nt, and the event

R_j(0,t)_∩L_1∩(B₆)cdepend only on these variables, we have_∀N_t

Px,y,j−1 L₂|R_j(0,t)_∩L₁∩(B₆)c_∩(n_t=N_t)

=Px,y,j−1 L₂|n_t=N_t

.

Since by Lemma 3,

for any value ofN_t, we deduce that

Combining (32), (33), (34), (35) and (36) into (24) we get

By Lemma 14, for any 2k_≤ j₁₆= j₂<2k+1

max

x,y∈A₂k₋1

Px,y,2k−1(S Ej1,j2

k (0,t))≤β 2 k

π

j₁3

π

j₂3 + C k

27k. (38)

S E_kj,j(0,t)_⊆SRj(0,t), therefore by Lemma 15, for any 2k≤ j<2k+1

max

x,y∈A₂k₋₁

Px,y,2k−1(S E_kj,j(0,t))_≤ C

j6. (39)

Combining (38) and (39) into (37) we conclude

max

x,y∈A₂k₋₁

Px,y,2k−1(S Ek(0,t))

≤ β_k2π2 X 2k_≤j

16=j2<2k+1

‚

1

j₁3 1

j₂3 + C k

27k

Œ

+ X

2k_≤j<2k+1

C

j6

≤



 βkπ

X

2k_≤j<2k+1

1 j3



  2

+ C k

25k.

3.4

Proof of Theorem 2

PROOF OFTHEOREM2. Define

f(t,M) = P( T

1≤k≤MS Ek(0,t))

(PT_1≤k≤M(S E_k(0)))2.

Setk0=K′(t). Then

f(t,M)_≤ 1

P(S Ek0(0))

2 M

Y

k=k0+1

P(S E_k(0,t)_|S E_k−1(0,t))

P(S Ek(0)|S Ek−1(0))2

. (40)

We start by estimating the first factor:

1

P(S E_k0(0))2 ≤C Y

0<k≤k0

1

Using the lower bound from Lemma 13 we get that this is bounded above by

For the second factor of (40), we use Lemma 13 to bound the denominator, together with Lemma 16 to bound the numerator. We get

n

Multiplying the two estimates we deduce that

f(t,M)_≤C(4(1+_|logt_|))3(log(|logt|+1)+2) (42)

for anyM. Since

Z 1

0

C(4(1+_|logt|))3(log(|logt|+1))+2d t<_∞,

4

Proof of Theorem 1

The proof of Theorem 1 follows from Theorem 2 and a second moment argument exactly as in[5]. We give it here for the sake of completeness.

PROOF OFTHEOREM1. Define

EM(t) =

\

1≤i≤M

S Ei(t),

T_M=_{t:t∈[0, 1]andE_M(t)occurs}

and

T=_∩∞₁ T_M.

Now we show thatT is contained in the union ofSexcand the countable set

Λ = (_∪n,mτ(_nm))_∪1.

Ift ∈ ∩∞1 TM thent∈Sexc. So if t∈T\Sexcthent is contained in the boundary ofTM for someM.

For anyM the boundary ofT_M is contained inΛ. Thus if t_∈T_\Sexcthent_∈Λand

T⊂Sexc∪Λ.

AsΛis countable ifT has dimension one with positive probability then so doesSexc.

By Theorem 2 there exists f(t)such that

Z 1

0

f(t)d t<_∞

and for allM

P(E_M(0,t)) (P(EM(0)))2

< f(M,t)< f(t). (43)

Let_L(_∗)denote Lebesgue measure on[0, 1]. Then we get

E(_L(T_M)2) = Z 1

0 Z 1

0

P(E_M(r,s))d r_×ds (44)

= Z 1

0 Z 1

0

P(EM(0,|s−r|))d r×ds (45)

≤

Z 1

0

2

Z 1

0

P(E_M(0,t))d t_×ds

≤ 2

Z 1

0

f(t)P(E_M(0))2d t (46)

≤ 2P(EM(0))2

Z 1

0

The equality (44) is true by Fubini’s theorem, (45) is true because

E_M(a,b) =E_M(b,a) =E_M(0,_|b₋a_|)

and (46) follows from (43).

By Jensen’s inequality ifh(x) =0 for allx _∈/Athen

E(h2)_≥ E(h) 2

P(A) . (48)

Then we get

2P(E_M(0))2 Z 1

0

f(t)d t _≥ E(_L(T_M)2) (49)

≥ E(L(TM)) 2

P(T_{M 6}=_;) (50)

≥ P(EM(0)) 2

P(TM6=;)

,

where (49) is a restatement of (47), and (50) follows from (48) with_L(T_M)andT_M6=_;in place ofhandA.

Thus for allM

P(T_M6=_;)_≥ 1

2R₀1f(t)d t

>0.

AsT is the intersection of the nested sequence of compact setsTM

P(T₆=_;) = lim

M→∞P(TM6=;) =Mlim→∞P(TM6=;)≥

1

2R₀1 f(t)d t .

Now we show that the dimensions ofT andSexcare one. By Lemma 5.1 of[11]for anyβ <1 there exists a random nested sequence of compact setsF_k⊂[0, 1]such that

P(r_∈F_k)_≥C(s_k)−β (51)

and

P(r,t _∈F_k)_≤C(s_k)−2β_|r₋t_|−β. (52) These sets also have the property that for any setT if

P(T_∩(_∩∞₁ F_k)₆=_;)>0 (53)

thenThas dimension at leastβ. We constructFkto be independent of the dynamical random walk.

So by (42), (51) and (52) we get

P(r,t∈TM∩FM)

P(r_∈T_M∩F_M)2 ≤C(1+|log|r−t| |)

Since

Z 1

0

C(1+_|logt_|)3|log(|logt|+1)|t−β d t<_∞,

the same second moment argument as above and (54) implies that with positive probability T satisfies (53). ThusT has dimensionβwith positive probability. As

T_⊂(Sexc_∩[0, 1])_∪Λ,

andΛis countable, the dimension of the set ofSexc∩[0, 1]is at leastβwith positive probability. By the ergodic theorem the dimension of the set ofSexcis at leastβ with probability one. As this holds for allβ <1 the dimension ofSexcis one a.s.

ƒ

4.1

Extending the technique

We end the paper with three remarks on the scope of the techniques in this paper.

1. The techniques in this paper can be extended to show other sets of exceptional times exist. For any two finite setsEandV, for which it is possible to visit every point inV without hitting E, there is a.s. an exceptional time tsuch that each point inV is visited infinitely often, while the setEis not visited at all.

2. Not every set can be avoided by the random walk, as shown in the following claim:

Claim 17. Let L be a subset ofZ2_{, with the property that there exits some M>0, such that every}

point inZ2is at distance at most M from some point in L. Then

P(_∃t_∈[0, 1] :_|{n: S_n(t)_∈/ L_}|<_∞) =0

i.e there are a.s. no exceptional times at which L is visited only finitely often.

Proof. First, we can reduce to the case that there is some (possible) path from 0 that missesL. Otherwise we can just drop points out ofLuntil such a path exists without ruining the desired condition on L

Second, we note that it is enough to prove that there are a.s. no times at which the random walkneverhits L. This follows from the Markov property of random walk, and the fact that changing finitely many moves is enough to make a walk that visitsLa finite number of times miss Lall together.

Let

Q_n=_{t_∈[0, 1] : S_i(t)_∈/L 0_{≤ ∀}i_≤n_}.

Then an exceptional time exists if and only if

The condition on Lensures that there is some ε >0, such that for any simple random walk on Z2, regardless of current position or history, there is a probability of at leastεof hitting L in the next 2M steps. Therefore there exists some constantsA>0 and 0<c<1 such that

P(0∈Qn)≤Acn

For anyi_≥0 define

mi=min{m≥0 :τ

(m)

i >1}.

Then

RE_i=nτ(_ij)omi j=0

is the set of times in[0, 1]in which thei₋thmove gets resampled. LetH_n=Sn_i=0RE_i. Then Hn is the set of all times at which one of the firstnmoves of the walk are re-sampled, and if

we orderH_n, then between two consecutive times in H_n, the firstnsteps of the walk do not change. ThusQ_{n 6}=_;if and only if there exists someτ_∈H_n for whichτ_∈Q_n. Since_{S_n(t)_}

behaves like a simple random walk for allt, we have, by linearity of expectation

E(#τ_∈H_n : τ_∈Q_n) =E(_|H_n|)P(0_∈Q_n)_≤2Anc−n Thus

P(Qn6=;)≤E(#τ∈Hn : τ∈Qn)≤2Anc−n

And consequently

P



 \

n≥0

Qn6=;



=0.

So no exceptional times exist.

3. We finish with an open question:

Is there a setA_⊂Z2_{such that bothAandA}c_{are infinite and almost surely there are exceptional}

times in which every element ofAis hit finitely often and every element ofAc is hit infinitely often?

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