theset
{y ∈ W s (x) | d(x, y) < δ}
isa onne ted urveoflength2δ
thendeneW δ s (x) = {y ∈ W s (x) | d(x, y) < δ}
tobethestablemanifoldofx
withlength2δ
. OtherwisewesaythatW δ s (x)
doesnotexist.Sin e
f
isnotinvertiblewe annotdene thelo alunstablemanifoldofx
intheusualway. Let
D − δ = {x | d(f −n (x), S) > δγ −n ∀n ≥ 0}.
ThissethaspositiveLebesguemeasureif
δ
istakensu ientlysmall,see[27℄.Itiseventruethat
µ SBR
∞
[
l=1
D l − − 1
= 1.
If
x = (x 1 , x 2 ) ∈ D δ −
thenforanyy = (y 1 , y 2 )
withy 1 = x 1
and|x 2 −y 2 | < δ
wehave
d(f −n (x), f −n (y)) < δγ −n
foralln ≥ 0
. We anthusdenetheunstablemanifoldof
x = (x 1 , x 2 ) ∈ D δ −
oflengthδ
tobethesetW δ u (x) = {y = (y 1 , y 2 ) | y 1 = x 1 , |x 2 − y 2 | < δ}.
If
x 6∈ D − δ
thenwesaythatW δ u (x)
doesnotexist.Lemma3.4.0.1. Let
ε > 0
bexedandI ⊂ (γ −1 +ε, 0.64)
. Thereisa onstantc
su hthatfor anyk ∈ N
andany{a i } ∞ i=0 ∈ {−1, 0, 1} N
thefollowingestimateisvalid.
Z
I
χ {λ||λ k +P ∞
i=k+1 a i λ i |<r} ≤ c(γ −1 + ε) −k r.
Let
γ
,κ
bexedandI ⊆ (0.5, 0.64)
anyintervalsu hthat{γ}×I×{κ} ⊂ P
,wheretheset
P
willbe hosenlater.Let
B r (t) = [t − r, t + r]
andletD(µ λ,s,x SBR , t) = lim inf
r→0
µ λ,s,x SBR (B r (t)) 2r
denotethelowerderivativeofthemeasure
µ λ,s,x SBR
the onditionalmeasureofµ λ SBR
withrespe ttothelo alstablemanifoldW s (x)
. ThepartitionofQ
intolo al stable manifolds is learly measurable so the onditional measures exist
andwe anusethemasfollows. Wewanttoprovethatfora.e.
λ
thereisasetΩ λ
su hthatµ λ SBR (Ω λ ) > 0
andZ
Ω λ
D(µ λ,s,x SBR | Ω λ , y)dµ λ,s,x SBR (y) < ∞
(3.1)holds for a.e.
x
. This implies that the measureµ λ,s,x SBR
restri ted to the setΩ λ
is absolutely ontinuousfora.e.x
. Sin ethe onditionalmeasures on the unstablemanifoldsareabsolutely ontinuouswithrespe ttoLebesguemeasure,this implies that
µ λ SBR | Ω λ
is absolutely ontinuous with respe t to Lebesguemeasure. Sin e
µ λ SBR (Ω λ ) > 0
, ergodi itythen impliesthatthis alsoholds forthemeasure
µ λ SBR
.Fatou'slemmaimpliesthatin ordertoprove(3.1)itsu estoprovethat
lim inf
r→o
1 r Z
Ω λ
µ λ,s,x SBR (Ω λ ∩ B r (y))dµ λ,s,x SBR (y) < ∞.
Wemayrewritethisas
lim inf
r→0
1 r
Z
Ω λ
Z
Ω λ
χ {|y 1 −z 1 |<r} dµ λ,s,x SBR (z)dµ λ,s,x SBR (y) < ∞.
(3.2)Wewill hoose a lassof fun tions
x : I → Q
su h thatx(λ) ∈ Ω λ
andprovethatthefollowingestimateisvalid
lim inf
r→0
1 r Z
I
Z
Ω λ
Z
Ω λ
χ {|y 1 −z 1 |<r} dµ λ,s,x(λ) SBR (z)dµ λ,s,x(λ) SBR (y)dλ < ∞.
(3.3)−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1
−1
−0.8
−0.6
−0.4
−0.2 0 0.2 0.4 0.6 0.8 1
γ=1.858, λ=0.54, k=0
Figure3.3: Someinverseimagesof
S v
.This thenimplies that
µ SBR ≪ ν
for a.e.λ ∈ I
. Instead ofproving(3.3) weusethat
µ SBR = ˆ µ SBR ◦ π −1
andprovetheequivalent onditionlim inf
r→0
1 r
Z
I
Z
Ω ˆ λ
Z
Ω ˆ λ
χ {|y 1 −z 1 |<r} dˆ µ λ,s,ˆ SBR x(λ) (ˆ z)dˆ µ λ,s,ˆ SBR x(λ) (ˆ y)dλ < ∞.
(3.4)Toprove(3.4)thesymboli oding
Σ λ,γ,κ
willbeused. Sin ewehaveγ
andκ
xedand varyλ
it isthedynami s taking pla ein the horizontaldire tionthatis ru ial. Itishen ethedynami albehaviouroftheverti al omponent
ofthesingularity
S v
thatisimportant. Below,wewill hooseasetP
inwhi hS v
behavesasifitdoesnotexist.One he ks withanumeri al al ulationthat for
γ = 1.858
,λ = 0.54
andκ = 0
there are only nitely many points in the omplete ba kwardorbit oftheverti alpie e ofthesingularity,
S v
. Numeri sareonlyused tosee thisinaneasyway. Itdoesnotinuen eontherigorousityoftheproof. Inthis ase,
when
κ = 0
, wehaveS v = {0}
. Figure3.3 and 3.4illustrate this byshowingthetwopossiblepathsof
{0}
startinginQ 1
. Thepointsintheba kwardorbitof
S v
are marked with×
in the gures. The dashed lines are the border ofthesets
f (Q 1 )
andf (Q −1 )
. Thearrowsshowshowthepointsaremappedbyf −1
. Ea hpathdrawnintheguresterminatesafternitelymanysteps,afterwhi hthere arenomoreinverseimages.
−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1
−1
−0.8
−0.6
−0.4
−0.2 0 0.2 0.4 0.6 0.8 1
γ=1.858, λ=0.54, k=0
Figure 3.4: Yet someinverseimagesof
S v
.The numeri al evaluations of the points are in Table 3.1 and Table 3.2.
Comparingthese valueswith
f (Q 1 ) = [1 − 2λ, 1] × [1 − γ, 1] = [ −0.08, 1] × [−0.858, 1], f (Q −1 ) = [ −1, 2λ − 1] × [−1, γ − 1] = [ −1, 0.08] × [−1, 0.858],
showsthattherearenomorepointsintheba kwardorbitof
S v
andthatthesepointsareboundedawayfromtheset
f (S)
. Thisallowustodrawthefollowingon lusion. Thereisanopenneighborhood
U
ofS v
su hthatf −N (U ) = ∅
forN
su ientlylarge.Bythe ontinuousdependen eontheparametersofthenitelymanypoints
in the ba kward orbit of
S v
, there exists an open ballP ⊂ {γ, λ, κ}
around(γ, λ, κ) = (1.858, 0.54, 0)
su h that theba kwardorbit ofS v
behavesin thesame way for any
(γ, λ, κ) ∈ P
in the followingsense. Forany(γ, λ, κ) ∈ P
the ba kward orbit of
S v
ontains nitely many pie es, ea h bounded awayfrom
f (S)
, and there areuniform numbersα > 0
andN > 0
su h thatU = ([ −α, α] × [−|κ|, |κ|])
satisesf −N (U ) = ∅
. This implies thatU ∩ Λ γ,λ,κ = ∅
forany
(γ, λ, κ) ∈ P
.Re allthat wehavetheparameters
γ
andκ
xedandI
isanintervalsu hthat
{γ} × I × {κ} ⊂ P
. PartitionI
into sub intervals{I t } p t=1
, su h that|I t | < 15 1 α
. This anbedonesothatp < 15|I| α + 1
. Forea ht = 1, . . . , p
xaiterate oordinates
numeri al
values ontainedin
0
00
0
0 f (Q 1 ) ∩ f(Q −1 )
−1
−1+λ
−1+γ λ γ
−0.8519
0.4618 f (Q −1 ) \ f(Q 1 )
−2
−1+2λ−λ 2 λ 2
−1+2γ−γ 2 γ 2
−0.7257
−0.2132 f (Q −1 ) \ f(Q 1 )
−3
−1+2λ−λ 3 λ 3
−1+2γ−γ 3 γ 3
−0.4919
−0.5766 f (Q −1 ) \ f(Q 1 )
−4
−1+2λ−λ 4 λ 4
−1+2γ−γ 4 γ 4
−0.05916
−0.7721 f (Q 1 ) ∩ f(Q −1 )
−5
−1+2λ−2λ 4 +λ 5 λ 5
−1+2γ−2γ 4 +γ 5 γ 5
−0.9614
0.04623 f (Q −1 ) \ f(Q 1 )
−6
−1+2λ−2λ 4 +2λ 5 −λ 6 λ 6
−1+2γ−γ 4 +2γ 5 −γ 6 γ 6
−0.9285
−0.4369 f (Q −1 ) \ f(Q 1 )
−7
−1+2λ−2λ 4 +2λ 5 −λ 7 λ 7
−1+2γ−γ 4 +2γ 5 −γ 7 γ 7
−0.8677
−0.6969 f (Q −1 ) \ f(Q 1 )
−8
−1+2λ−2λ 4 +2λ 5 −λ 8 λ 8
−1+2γ−γ 4 +2γ 5 −γ 8 γ 8
−0.7549
−0.8369 f (Q −1 ) \ f(Q 1 )
−9
−1+2λ−2λ 4 +2λ 5 −λ 9 λ 9
−1+2γ−γ 4 +2γ 5 −γ 9 γ 9
−0.5462
−0.9122 f (Q −1 ) \ f(Q 1 )
−10
−1+2λ−2λ 4 +2λ 5 −λ 10 λ 10
−1+2γ−γ 4 +2γ 5 −γ 10 γ 10
−0.1596
−0.9528 f (Q −1 ) \ f(Q 1 )
−11
−1+2λ−2λ 4 +2λ 5 −λ 11 λ 11
−1+2γ−γ 4 +2γ 5 −γ 11 γ 11
0.5563
−0.9746 Q \ (f(Q 1 ) ∪ f(Q −1 ))
Table3.1: ThepointsinFigure 3.3.
iterate oordinates
numeri al
values ontainedin
0
00
0
0 f (Q 1 ) ∩ f(Q −1 )
−1
−1+λ
−1+γ λ γ
−0.8519
0.4618 f (Q −1 ) \ f(Q 1 )
−2
−1+2λ−λ 2 λ 2
−1+2γ−γ 2 γ 2
−0.7257
−0.2132 f (Q −1 ) \ f(Q 1 )
−3
−1+2λ−λ 3 λ 3
−1+2γ−γ 3 γ 3
−0.4919
−0.5766 f (Q −1 ) \ f(Q 1 )
−4
−1+2λ−λ 4 λ 4
−1+2γ−γ 4 γ 4
−0.05916
−0.7721 f (Q 1 ) ∩ f(Q −1 )
−5
−1+2λ−λ 5 λ 5
−1+2γ−γ 5 γ 5
0.7423
−0.8773 Q \ (f(Q 1 ) ∪ f(Q −1 ))
Table3.2: ThepointsinFigure3.4.
λ t ∈ I t
. Thefollowinglemmawillprovidesu ient ontrolwhen hangingtheparameter
λ
.Lemma3.4.0.2. Forany
t
andanyλ, λ ′ ∈ I t
the symboli spa esΣ λ,γ,κ
andΣ λ ′ ,γ,κ
oin ide.Proof. Apoint
x = (x ˆ 1 , x 2 , x 3 ) ∈ ˆ Q
liesinΛ ˆ λ t
ifandonlyifthereisasequen e{i n } n∈Z
su hthatx 1 = 1 − λ t
λ t
X ∞ n=1
i −n λ n t , x 2 = γ − 1 γ
X ∞ n=0
i n γ −n , x 3 = 1 − τ τ
X ∞ n=1
i −n τ n
and
f ˆ λ n t (ˆ x) ∈ ˆ Q i n
foralln ∈ Z
. Letx = (x ˆ 1 , x 2 , x 3 ) ∈ ˆ Λ λ t
. Weshowthat forany
λ ′ ∈ I t
thereisapointx ˆ ′ = (x ′ 1 , x ′ 2 , x ′ 3 ) ∈ ˆ Λ λ ′
su hthatthe orresponding sequen e{i ′ n } n∈Z
satisesi ′ n = i n
for alln ∈ Z
. InthiswaywedeneamapΞ λ t ,λ ′ : ˆ Λ λ t → ˆ Λ λ ′
byΞ λ t ,λ ′ : ˆ x 7→ ˆx ′
.Itsu estoshowthatthepoint
x ˆ ′ = (x ′ 1 , x ′ 2 , x ′ 3 )
dened byx ′ 1 = 1 − λ ′
λ ′ X ∞ n=1
i −n (λ ′ ) n , x ′ 2 = γ − 1 γ
X ∞ n=0
i n γ n , x ′ 3 = 1 − τ τ
X ∞ n=1
i −n τ n
satises
f ˆ λ n ′ (ˆ x ′ ) ∈ ˆ Q i n
foralln ∈ Z
. Thenthisimpliesthatx ˆ ′ ∈ ˆ Λ λ ′
. A hangeof
λ
hasonlyinuen eonthese ond oordinateoff (ˆ ˆ x)
. Sin ethelo alstablemanifoldsare parallel and oriented in the dire tion of the se ond oordinate
it su esto he k that when hanging
λ
, these ond oordinate nevermoveovertheverti aldis ontinuity
S v
. Wedothisbyanestimateofthederivatived
dλ
1−λ λ
P ∞
n=0 i −n λ n
. Asimple al ulationgivesthat
d dλ
1 − λ λ
X ∞ n=0
i −n λ n ≤
1 λ 2
X ∞ n=0
i n λ n +
1 − λ λ
X ∞ n=1
ni n λ n−1
≤ 1 λ 2
X ∞ n=0
λ n + 1 − λ λ
X ∞ n=1
nλ n−1 = 1 + λ
λ 2 (1 − λ) < 15,
if
1
2 < λ < 3 4
. Thisimpliesthat|x 1 − x ′ 1 | ≤ sup
λ
d dλ
1 − λ λ
X ∞ n=0
i −n λ n
|λ t − λ ′ | < 15|I t | ≤ α.
Thismeansthat
ˆ x ′
doesnot rosstheverti alpie eofthesingularityandhen e stayson thesame side ofthe singularityasx ˆ
. Similarlyoneshowsthat anyiterate
f ˆ λ n ′ (ˆ x ′ )
ofstaysonthesamesideofthesingularityasf ˆ λ n t (ˆ x)
.Remark. The partition of
I
intosubintervalsisarbitrary soinfa t the sym-boli spa es oin ide for anyλ, λ ′ ∈ I
.We have shown that the symboli spa e
Σ λ,γ,κ
does not hange whenλ
varies. Wealsoneedtoestimatehowthemeasure
µ ˆ λ SBR
hanges.Lemma 3.4.0.3. There is a onstant
c 1 > 0
su h that for anyt
and anyλ, λ ′ ∈ I t
c −1 1 µ ˆ λ SBR t (C λ t ) ≤ ˆµ λ SBR (C λ ) ≤ c 1 µ ˆ λ SBR t (C λ t ),
(3.5)forany ylinder setoftheform
C λ = k [i k i k+1 · · · i n ] n = T n
j=k f ˆ λ −j ( ˆ Q i j )
,k < n
.Proof. Sin ethereareonlynitelymanyinverseimagesoftheset
S v
,allotherinverseimagesofthesingularitywill onsistofahorizontalline. As
λ
variesoverI
thesehorizontallinesarenot hanged,onlytheinverseimagesofS v
hanges.Thereisthusa onstant
c 1
,independentoft
,su hthatforanyλ ∈ I t
andanyylindersetoftheform
C λ = k [i k i k+1 · · · i n ] n = T n
j=k f ˆ λ −j ( ˆ Q i j )
,0 ≤ k < n
wehave
c −1 1 ˆ ν(C λ t ) ≤ ˆν(C λ ) ≤ c 1 ˆ ν(C λ t ).
Espe ially
c −1 1 ν( ˆ ˆ f λ −m t (C λ t )) ≤ ˆν( ˆ f λ −m (C λ )) ≤ c 1 ν( ˆ ˆ f λ −m t (C λ t ))
for anym ∈ N
andso
c −1 1 µ ˆ λ SBR t (C λ t ) ≤ ˆµ λ SBR (C λ ) ≤ c 1 µ ˆ λ SBR t (C λ t ),
for any ylinder set of the form
C λ = k [i k i k+1 · · · i n ] n = T n
j=k f ˆ λ −j ( ˆ Q i j )
withk < n
.Remark. Lemma 3.4.0.3 is also valid for the onditional measures on the
stable manifold.
Sin e the entropy of the onditional measures are a.s. equal that of the
measure, the Shannon-M Millan-Breiman theorem implies that given
ε > 0
,thereexists anumber
n 0 (λ, ˆ x, ε)
su hthatˆ
µ λ,s,ˆ SBR x n ˆ
y | ˆµ λ,s,ˆ SBR x ( ˆ R q λ (ˆ y)) < e −q log(γ−ε) , q > n 0 (λ, ˆ x, ε) o
> 1 − ε.
Lusin'sTheoremimpliesthatthereisanumber
n 1 (ε)
andasetΩ ˆ 0,λ t
su hthatn 1 (ε) ≥ n 0 (λ t , ˆ x, ε)
whenx ˆ ∈ ˆ Ω 0,λ t
andµ ˆ λ SBR t ( ˆ Ω 0,λ t ) > 1 − ε
. WeputΩ ˆ λ t = n
ˆ
y ∈ ˆ Ω 0,λ t ∩ ˆ W s (ˆ x) | ˆx ∈ Ω 0,λ t
andˆ
µ λ,s,ˆ SBR x ( ˆ R q λ (ˆ y)) < e −q log(γ−ε) , q > n 1 (ε) o .
Then
µ ˆ λ SBR t ( ˆ Ω λ t ) > 1 − 2ε
.Put
Ω ˆ λ = Ξ λ t ,λ ( ˆ Ω t )
forλ ∈ I t
. ByLemma3.4.0.3itfollowsthat ify ˆ ∈ ˆ Ω λ
then
ˆ
µ λ SBR ( ˆ R q λ (ˆ y)) < c 1 e −q log(γ−ε) , q > n 1 (ε)
and
ˆ
µ λ SBR ( ˆ Ω λ ) > c −1 1 µ ˆ λ SBR t (Ω λ t ) > c −1 1 (1 − 2ε).
Takea
λ 0 ∈ I
andx ˆ λ 0 ∈ ˆ Ω λ 0
. Denex(λ ˆ 0 ) = ˆ x λ 0
andforλ ∈ I
denex(λ) ˆ
sothat
ρ −1 λ,γ,κ (ˆ x(λ)) = ρ −1 λ 0 ,γ,κ (ˆ x(λ 0 ))
. Thenx ˆ
is ontinuous. ByLemma3.4.0.1T i,t,k = Z
I t
Z
Ω ˆ λ ∩ ˆ R k i1 (λ)
Z
Ω ˆ λ ∩ ˆ R k i−1 (λ)
χ {|y 1 −z 1 |<r} dˆ µ λ,s,ˆ SBR x(λ) (ˆ z)dˆ µ λ,s,ˆ SBR x(λ) (ˆ y)dλ
≤ c 2 1
Z
I t
Z
Ω ˆ λt ∩ ˆ R k i1 (λ t )
Z
Ω ˆ λt ∩ ˆ R k i−1 (λ t )
χ {|Ξ λt,λ (ˆ y) 1 −Ξ λt,λ (ˆ z) 1 |<r}
dˆ µ λ SBR t ,s,ˆ x(λ t ) (ˆ z)dˆ µ λ SBR t ,s,ˆ x(λ t ) (ˆ y)dλ
= c 2 1 Z
Ω ˆ λt ∩ ˆ R k i1 (λ t )
Z
Ω ˆ λt ∩ ˆ R k i−1 (λ t )
Z
I p
χ {|Ξ λt,λ (ˆ y) 1 −Ξ λt,λ (ˆ z) 1 |<r}
dλdˆ µ λ SBR t ,s,ˆ x(λ t ) (ˆ z)dˆ µ λ SBR t ,s,ˆ x(λ t ) (ˆ y)
≤ c 2 (γ −1 + ε) −k r ˆ µ λ SBR t ,s,ˆ x(λ t ) ( ˆ R k i1 (λ t ))ˆ µ λ SBR t ,s,ˆ x(λ t ) ( ˆ R k i−1 (λ t ))
≤ c 2 r(γ −1 + ε) −k (γ − ε) −k µ ˆ λ SBR t ,s,ˆ x(λ t ) ( ˆ R k i1 (λ t )).
Sin e
Λ ˆ λ × ˆ Λ λ = [ ∞ k=0
[
i
R ˆ k i1 (λ) × ˆ R i−1 k (λ)
we anpro eedasfollows
Z
I
Z
Ω ˆ λ
Z
Ω ˆ λ
χ {|y 1 −z 1 |<r} dˆ µ λ,s,ˆ SBR x(λ) (z)dˆ µ λ,s,ˆ SBR x(λ) (y)dλ
= X p t=1
X ∞ k=0
X
i
T i,t,k
≤ X p t=1
X ∞ k=0
X
i
c 3 r(γ −1 + ε) −k (γ − ε) −k µ ˆ λ SBR t ,s,ˆ x(λ t ) ( ˆ R k i1 (λ t ))
≤ X p t=1
X ∞ k=0
c 3 r(γ −1 + ε) −k (γ − ε) −k ≤ X p t=1
c 4 r = c 5 r.
Hen e
lim inf
r→0
1 r
Z
I
Z
Ω ˆ λ
Z
Ω ˆ λ
χ {|y 1 −z 1 |<r} dˆ µ λ,s,ˆ SBR x(λ) (ˆ z)dˆ µ λ,s,ˆ SBR x(λ) (ˆ y)dλ ≤ c 5 .
(3.6)Thisisindependentofthe hoi e of