transverse stability of periodic waves in water-wave models...transverse stability of periodic waves...
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Transverse stability of periodic
waves in water-wave models
Mariana Haragus
Institut FEMTO-ST and LMB
Universite Bourgogne Franche-Comte, France
ICERM, April 28, 2017
Water-wave problem
gravity/gravity-capillary waves
� three-dimensional inviscid fluid layer
� constant density
� gravity/gravity and surface tension
� irrotational flow
Water-wave problem
x
y
z
y = 0 (flat bottom)
y = h + η(x , z, t)
(free surface)
Domain
Dη = {(x , y , z) : x , z ∈ R, y ∈ (0, h + η(x , z, t))}
� depth at rest h
Euler equations
Laplace’s equation
φxx
+ φyy
+ φzz
= 0 in Dη
boundary conditions
φy
= 0 on y = 0
ηt
= φy
− ηx
φx
− ηz
φz
on y = h + η
φt
= −1
2(φ2
x
+ φ2y
+ φ2z
) − gη +σ
ρK on y = h + η
� velocity potential φ; free surface h + η
� mean curvature K =
[ηx√
1+η2x+η2
z
]
x
+
[ηz√
1+η2x+η2
z
]
z� parameters ρ, g , σ, h
Euler equations
moving coordinate system, speed −
dimensionless variables
� characteristic length h
� characteristic velocity
parameters
� inverse square of the Froude number α =gh
2
� Weber number β =σ
ρh 2
Euler equations
φxx
+ φyy
+ φzz
= 0 for 0 < y < 1 + η
φy
= 0 on y = 0
φy
= ηt
+ ηx
+ ηx
φx
+ ηz
φz
on y = 1 + η
φt
+ φx
+ 12
(φ2x
+ φ2y
+ φ2z
)+ αη − βK = 0 on y = 1 + η
Euler equations
φxx
+ φyy
+ φzz
= 0 for 0 < y < 1 + η
φy
= 0 on y = 0
φy
= ηt
+ ηx
+ ηx
φx
+ ηz
φz
on y = 1 + η
φt
+ φx
+ 12
(φ2x
+ φ2y
+ φ2z
)+ αη − βK = 0 on y = 1 + η
difficulties
� variable domain (free surface)
� nonlinear boundary conditions
very rich dynamics
� symmetries, Hamiltonian structures
� many particular solutions
Focus on . . .
traveling periodic 2D waves
transverse stability/instability
analytical results
long-wave models
Two-dimensional periodic waves
exist in different parameter regimes
β13
1
α
Two-dimensional periodic waves
transverse (in)stability
β13
1
α
Large surface tension
transverse linear instability
� longitudinal co-periodic perturbations
� transverse periodic perturbations
Euler equations
[H., 2015]
Transverse instability problem
Transverse spatial dynamics
U
z
= DU
t
+ F (U)
�U(x , z, t), D linear operator, F nonlinear map
� a periodic wave U∗(x) is an equilibrium
z
x
Transverse linear instability
Transverse spatial dynamics
U
z
= DU
t
+ F (U)
U∗(x) is transversely linearly unstable if the linearized system
U
z
= DU
t
+ LU , L = F
′(U∗)
possesses a solution of the form U(x , z, t) = eλtVλ(x , z)
with λ ∈ C, Reλ > 0, Vλ bounded function.
Hypotheses
1 the system U
z
= DU
t
+ F (U) is reversible/Hamiltonian;
2 the linear operator L = F
′(U∗) possesses a pair of
simple purely imaginary eigenvalues ±iκ∗;
3 the operators D and L are closed in X with D(L) ⊂ D(D);
Main result
Theorem
1 For any λ ∈ R sufficiently small, the linearized system
U
z
= DU
t
+ LU
possesses a solution of the form U(·, z, t) = eλtVλ(·, z)
with Vλ(·, z) ∈ D(L) a periodic function in z .
2 U∗ is transversely linearly unstable.
[Godey, 2016; see also Rousset & Tzvetkov, 2010]
Euler equations
Hamiltonian formulation of the 3D problem:
U
z
= DU
t
+ F (U)
� boundary conditions
φy
= b(U)t
+ g(U) on y = 0, 1
(e.g. [Groves, H., Sun, 2002])
Periodic waves
β >1
3, α = 1 + ǫ , ǫ small
model: Kadomtsev-Petviashvili-I equation instability
∂x∂tu =∂x∂x(∂2xu + u +
1
2u2)−∂2
yu
[H.; Johnson & Zumbrun; Hakkaev, Stanislavova & Stefanov, . . . ]
Periodic waves
β >1
3, α = 1 + ǫ , ǫ small
model: Kadomtsev-Petviashvili-I equation instability
∂x∂tu =∂x∂x(∂2xu + u +
1
2u2)−∂2
yu
[H.; Johnson & Zumbrun; Hakkaev, Stanislavova & Stefanov, . . . ]
The Euler equations possess a one-parameter family of
symmetric periodic waves
ηǫ,a(x) = εpa
(ε1/2x, ε), ϕǫ,a(x) = ε1/2
q
a
(ε1/2x, ε)
p
a
(ξ, 0) = ∂ξqa(ξ, 0), pa(ξ, 0) satisfies the Korteweg de
Vries equation[Kirchgassner, 1989]
Linearized system
linearized system (rescaled)
U
z
= DεUt
+ DFε(ua)U
� boundary conditions
φy
=Dbε(ua)Ut
+ Dgε(ua)U on y = 0, 1
Linearized system
linearized system (rescaled)
U
z
= DεUt
+ DFε(ua)U
� boundary conditions
φy
=Dbε(ua)Ut
+ Dgε(ua)U on y = 0, 1
linear operator Lε := DFε(ua)
� boundary conditions
φy
= Dgε(ua)U on y = 0, 1
� space of symmetric functions (x → −x)
Xs = H1e (0, 2π)× L
2e(0, 2π)× H
1o ((0, 2π)× (0, 1))× L
2o((0, 2π)× (0, 1))
Linear operator Lε
Lε = L0ε + L1
ε L0ε
η
ω
φ
ξ
=
ω
β−εk
2aβηxx + (1 + ǫ)η − kaφx |y=1
ξ
−εk2aφxx − φyy
, L1ε
η
ω
φ
ξ
=
g1
g2
G1
G2
g1 =(1 + εk2a η
2ax )
1/2
β
(ω +
1
1 + εηa
∫1
0yφay ξ dy
)−
ω
β
g2 =
∫ 1
0
{εk
2aφaxφx −
φayφy
(1 + εηa)2+
εφ2ayη
(1 + εηa)3−
ε3k2a y2η2
axφayφy
(1 + εηa)2−
ε3k2a y2ηaxφ
2ayηx
(1 + εηa)2+
ε3k2a y2η2
axφ2ayη
(1 + εηa)3
+
[εk
2a yφayφx + εk
2a yφaxφy −
2ε2k2a y2ηaxφayφy
1 + εηa−
ε2k2a y2φ2
ayηx
1 + εηa+
ε3k2a y2ηaxφ
2ayη
(1 + εηa)2
]
x
}dy
+ εk2aβηxx − εk
2aβ
[ηx
(1 + ε3k2aη2ax )
3/2
]
x
G1 = −εηaξ
1 + εηa+
(1 + ε3k2aη2ax )
1/2
β(1 + εηa)
(ω +
1
1 + εηa
∫1
0yφay ξ dy
)yφay
G2 =
[εηaφ
(1 + εηa)+
εφaη
(1 + ηa)2
]
yy
− ε2k2a [ηaφx + φaxη − yφay ηx − yηaxφy ]x
+ ε2k2a
[yηaxφx + yφaxηx +
ε2y2η2axφayη
(1 + εηa)2−
εy2η2axφy
1 + εηa−
2εy2ηaxφayηx
1 + εηa
]
y
Check hypotheses . . .
Main difficulty: spectrum of Lε . . .
operator with compact resolvent −→ pure point spectrum
spectral analysis
|λ| ≥ λ∗
|λ| ≤ λ∗
|λ| ≤ εℓ∗
Key step
Reduction to a scalar operator Bε,ℓ in L2o(0, 2π)
� scaling λ = εℓ, ω = εω, ξ = εξ
� decomposition φ(x , y) = φ1(x) + φ2(x , y)
� λ = εℓ eigenvalue iff Bε,ℓφ1 = 0
Bε,ℓφ1 =
(β −
1
3
)k
4a
φ1xxxx − k
2a
φ1xx + ℓ2(1 + ǫ)φ1 − 3k2a
(Pa
φ1x)x + . . .
. . . . . . . . .
ω =β
(1 + ǫ3η⋆2x )1/2
(η†+ ikη) −
1
1 + ǫη⋆
∫ 1
0yΦ
⋆y ξdy,
ξ = (1 + ǫη⋆)(Φ
†+ ikΦ) − ǫyΦ
⋆y (η
†+ ikη)
(1 + ǫ)
ǫ2η −
1
ǫ2Φx |y=1 −
1
ǫβηxx − ikβ(h
ǫ1 + ikη) = h
ǫ2
−1
ǫΦxx −
1
ǫ2Φyy − ik(H
ǫ1 + ikΦ) = H
ǫ2 ,
hǫ2 = ω
†− g
ǫ2 ,
Hǫ2 = ξ
†− G
ǫ2
hǫ1 =
ω
β− ikη
= −1
β(1 + ǫη⋆)
∫ 1
0yΦ
⋆y [−ǫyΦ
⋆y (ikη + η
†) + (1 + ǫη
⋆)(ikΦ + Φ
†)]dy
+
(1
(1 + ǫ3η⋆2x )1/2
− 1
)ikη +
η†
(1 + ǫ3η⋆2x )1/2
,
Hǫ1 = ξ − ikΦ
= (1 + ǫη⋆)Φ
†+ ikǫη
⋆Φ − ǫyΦ
⋆y (η
†+ ikη).
Bǫ(η,Φ) = −ǫηx + B
ǫ0 + B
ǫ1 ,
Bǫ0 =
ǫη⋆Φy
1 + ǫη⋆+
ǫΦ⋆y η
(1 + ǫη⋆)2
∣∣∣∣∣y=1
,
Bǫ1 = ǫ
2η⋆x Φx + ǫ
2Φ⋆x ηx +
ǫ4η⋆2x Φ⋆
y η
(1 + ǫη⋆)2−
ǫ3η⋆2x Φy
1 + ǫη⋆−
−Φyy + q2Φ = ǫ2(Hǫ2 + ikHǫ
1 ), 0 < y < 1
Φy = 0, y = 0
Φy −ǫµ2Φ
1 + ǫ + βq2= −
ǫ3iµ(hǫ2 + ikβhǫ1 )
1 + ǫ + βq2+ B
ǫ0 + B
ǫ1 , y = 1
G(y, ζ) =
cosh qy
cosh q
(1 + ǫ + βq2) cosh q(1 − ζ) + (ǫµ2/q)
q2 − (1 + ǫ + βq2)q tanh q − ǫ
cosh qζ
cosh q
(1 + ǫ + βq2) cosh q(1 − y) + (ǫµ2/q)
q2 − (1 + ǫ + βq2)q tanh q −
. . . . . . . . .
Φ1 =1 + ǫ
ǫ2(k2(1 + ǫ) + µ2 + (β − 1/3)µ4)×
{∫ 1
0ǫ2(ξ
†− iµG
ǫ2,2 + ikH
ǫ1 )dζ − ǫq
2∫ 1
0pǫ2 dζ
−ǫ3iµ(hǫ2 + ikβhǫ1 )
1 + ǫ + βq2+
ǫ2µ2 pǫ2 |ζ=1
1 + ǫ + βq2
},
Φ2 = −
∫1
0G1(ξ
†− iµG
ǫ2,2 + ikH
ǫ1 )dζ −
∫1
0G1ζ G
ǫ2,1dζ +
∫1
0(ǫk
2+ µ
2)G1p
ǫ2 dζ + ǫp
ǫ2
− G1|ζ=1
(−
ǫiµ(hǫ2 + ikβhǫ1 )
1 + ǫ + βq2+
µ2pǫ2 |ζ=1
1 + ǫ + βq2
),
Φ = −
∫ 1
0Gǫ
2(ξ
†− ıµG
ǫ2,2 + ıkH
ǫ1 )dζ −
∫ 1
0Gζǫ
2Gǫ2,1dζ +
ǫ3 ıµG |ζ=1(hǫ2 + ıkβhǫ1 )
1 + ǫ + βq2+
∫ 1
0ǫq
2Gp
ǫ2 dζ + ǫp
ǫ2 −
ǫ2µ2
1
∫1
0ǫq
2Gp
ǫ2 dζ + ǫp
ǫ2 −
ǫ2µ2G |ζ=1 pǫ2 |ζ=1
1 + ǫ + βq2
=
∫ 1
0Gǫp
ǫ2ζζdζ − ǫG |ζ=1 p
ǫ2ζ |ζ=1 =
∫ 1
0Gǫ
2(G
ǫ2,0)ζζdζ − G |ζ=1B
ǫ0 ,
Φ1 + Φ2 = −
∫1
0Gǫ
2(ξ
†− ıµG
ǫ2,2 + ıkH
ǫ1 )dζ −
∫1
0Gζǫ
2Gǫ2,1dζ
+ǫ3 ıµG |ζ=1(h
ǫ2 + ıkβhǫ1 )
1 + ǫ + βq2+
∫1
0ǫq
2Gp
ǫ2 dζ + ǫp
ǫ2 −
ǫ2µ2G |ζ=1pǫ2 |ζ=1
1 + ǫ + βq2,
. . . . . . . . .
ıµhǫ2 = ıµω
†+ ıµF
[−
1
ǫ2
∫1
0
{ǫΦ
⋆x Φx −
Φ⋆y Φy
(1 + ǫη⋆)2+
ǫΦ⋆2y η
(1 + ǫη⋆)3−
ǫ3y2η⋆2x Φ⋆
y Φy
(1 + ǫη⋆)2−
ǫ3y2η⋆x Φ⋆2
y ηx
(1 + ǫη⋆)2+
ǫ4y
(1
+µ2
ǫF
[∫1
0
{yΦ
⋆y Φx + yΦ
⋆x Φy −
2ǫy2η⋆x ΦyΦ
⋆y
1 + ǫη⋆−
ǫy2Φ⋆2y ηx
1 + ǫη⋆+
ǫ2y2η⋆x Φ⋆2
y η
(1 + ǫη⋆)2
}dy
]−
βµ2
ǫF
[
(1 +
F−1
[ǫıµhǫ2
1 + ǫ + βq2
]= −F
−1[
1
1 + ǫ + βq2F [(Φ
⋆1xΦ1x )x ]
]+ F
−1
[µ2
1 + ǫ + βq2F
[∫1
0yΦ
⋆x Φ2y dy
]]
+
{F
−1
[−
1
1 + ǫ + βq2F
[∫ 1
0
(Φ⋆2xΦ1x + Φ
⋆x Φ2x −
Φ⋆y Φy
ǫ(1 + ǫη⋆)2+
Φ⋆2y η
(1 + ǫη⋆)3
−ǫ2y2η⋆2
x Φ⋆y Φy
(1 + ǫη⋆)2−
ǫ2y2η⋆x Φ
⋆2y ηx
(1 + ǫη⋆)2+
ǫ3y2η⋆x Φ⋆2
y η
(1 + ǫη⋆)3
)dy
]
−ıµ
1 + ǫ + βq2F
[∫1
0
(yΦ
⋆y Φx −
2ǫy2η⋆x Φ
⋆y Φy
1 + ǫη⋆−
ǫy2Φ⋆2y ηx
1 + ǫη⋆+
ǫ2y2η⋆x Φ
⋆2y η
(1 + ǫη⋆)2
)dy
]
+βıµ
1 + ǫ + βq2F
[ηx
(1 + ǫ3η⋆2x )3/2
− ηx
]]}
x
+ F−1
[ǫıµω†
1 + ǫ + βq2
]
= −F−1[
1
1 + ǫ + βq2F [(Φ
⋆1xΦ1x )x ]
]+ F
−1
[µ2
1 + ǫ + βq2F
[∫1
0yΦ
⋆x Φ2y dy
]]
+ (L(ǫΦ1x ,Φ2x ,Φ2y , ǫ2η, ǫ
4ηx ))x + ǫ
−1/2(L(ǫΦx , ǫ
2Φ2y , ǫ
4η, ǫ
3ηx ))x + ǫ
1/2L(ω
†),
. . . . . . . . .
F−1
[µ2
1 + ǫ + βq2F
[∫1
0yΦ
⋆x Φ2y dy
]]= F
−1
[µ2
1 + ǫ + βq2F
[Φ⋆1xΦ2|y=1 −
∫1
0Φ⋆1xΦ2dy +
∫1
0yΦ
⋆2xΦ2y
=
[F
−1
[µ1/2
1 + ǫ + βq2µ1/2
F [Φ⋆1xΦ2|y=1 ] −
1
1 + ǫ + βq2
∫1
0(Φ
⋆1xΦ2)xdy +
µ
1 + ǫ + βq2
∫1
0yΦ
⋆2xΦ2y dy
]]
x
= ǫ−1/4
(L(Φ2))x + (L(Φ2,Φ2x , ǫ1/2
Φ2y ))x ,
F−1
[ǫıµhǫ2
1 + ǫ + βq2
]= −F
−1[
1
1 + ǫ + βq2F [(Φ
⋆1xΦ1x )x ]
]+ ǫ
−1/4(L(Φ2))x
+ ǫ−1/2
(L(ǫΦx , ǫ2Φ2y , ǫ
4η, ǫ
3ηx )x + (L(ǫΦ1x ,Φ2,Φ2x ,Φ2y , ǫ
2η, ǫ
4ηx )x + H.
F−1
[ǫıµ.ıkhǫ1
1 + ǫ + βq2
]= (L(Φ2, ǫ
2η))x + ǫ
2k2(L(Φ1))x + H, F
−1
[µ2 pǫ2 |ζ=1
1 + ǫ + βq2
]= ǫ
−1/4(L(Φ2, ǫη))x
F−1[(ǫk
2+ µ
2)
∫ 1
0pǫ2 dζ
]= k
2L(ǫΦ2, ǫ
2η) + (L(Φ2,Φ2x , ǫη, ǫηx ))x
∫ 1
0(ξ
†− (G
ǫ2,2)x + ıkH
ǫ1 )dζ = (η
⋆Φ1x )x + (Φ
⋆1xη)x + (L(Φ2x ,Φ2y , ǫη, ǫηx ))x +
(β − 1/3)Φ1xxxx − Φ1xx + k2(1 + ǫ)Φ1 = (η
⋆Φ1x )x + (Φ
⋆1xη)x + F
−1[
1
1 + ǫ + βq2F [(Φ
⋆1xΦ1x )x ]
]
+ (L(ǫ1/2
Φ1x , ǫ−1/4
Φ2,Φ2x ,Φ2y , ǫ3/4
η, ǫηx ))x + k2[L(ǫΦ1, ǫΦ2, ǫ
2η) + ǫ
2L(Φ1)x ] + H,
η = F−1
[ıµΦ1
1 + ǫ + βq2
]+ L(ǫΦ1x , ǫ
3/4Φ2,Φ2x ,Φ2y , ǫ
3η, ǫ
7/2ηx ) + k
2ǫ3L(Φ1) + H.
Locate eigenvalues
|λ| ≤ εℓ∗
two simple eigenvalues ±iεκε
� Bε,ℓ small relatively bounded perturbation of B0,ℓ
B0,ℓ = k
2a
∂x
A ∂x
+ ℓ2 A =
(β −
1
3
)k
2a
∂xx
− 1 − 3Pa
Locate eigenvalues
|λ| ≤ εℓ∗
two simple eigenvalues ±iεκε
� Bε,ℓ small relatively bounded perturbation of B0,ℓ
B0,ℓ = k
2a
∂x
A ∂x
+ ℓ2 A =
(β −
1
3
)k
2a
∂xx
− 1 − 3Pa
� spectrum of ∂x
A∂x
is known (KP-I):
one simple negative eigenvalue −ω2a
� perturbation arguments . . . . . .
Critical surface tension
transverse linear instability
� longitudinal co-periodic perturbations
� transverse periodic perturbations
5th order KP model
[H. & Wahlen, 2017]
A 5th order KP model
∂t
∂x
u = ∂2x
(∂4x
u + ∂2x
u +1
2u
2
)+ ∂2
y
u
traveling generalized solitary waves
� solutions of the Kawahara equation
∂t
u = ∂x
(∂4x
u + ∂2x
u − u +1
2u
2
)
Periodic traveling waves
small periodic traveling waves: a two-parameter family
ϕa, (x) = p
a, (ka, x)
� depend analytically upon (a, c) ∈ (−a0, a0)× (−c0, c0)
� ka,c = k0(c) + ck(a, c),
k0(c) =(
1+√
1+4c2
)1/2, k(a, c) =
∑n≥1 k2n(c)a
2n
� pa,c(z) = ac cos(z) + c∑
m,n pn,m(c)ei(n−m)zan+m,
(n,m ≥ 0, n + m ≥ 2, n − m 6= ±1)
� explicit Taylor expansions for k(a, c), pn,m(c)
[Lombardi, 2000]
Transverse instability problem
one-dimensional periodic wave u∗
u∗ is transversely linearly unstable if the linearized equation
∂t
∂x
u = ∂2x
(∂4x
u + ∂2x
u − u + u∗u
)+ ∂2
y
u
possesses a solution of the form u(t, x , y) = eλtv(x , y) ,
for some Reλ > 0
(v belongs to the set of the allowed perturbations)
Transverse instability problem
linearized equation
∂t
∂x
u = A∗u + ∂2y
u , A∗ = ∂2x
(∂4x
+ ∂2x
− + u∗
)
� Fourier transform in y
∂t
∂x
u = A∗u − ω2u
Transverse instability problem
linearized equation
∂t
∂x
u = A∗u + ∂2y
u , A∗ = ∂2x
(∂4x
+ ∂2x
− + u∗
)
� Fourier transform in y
∂t
∂x
u = A∗u − ω2u
u∗ is transversely unstable if there exists a solution of the
form u(t, x) = eλtv(x) , for some Reλ > 0, and ω ∈ R
∗
�v ∈ H , a space of functions depending upon the longitudinal
spatial variable x , e.g., H = L
2(R) or H = L
2(0, L), and
λ∂x
v = A∗v − ω2v
Transverse instability problem
for some Reλ > 0 and ω ∈ R∗ , there exists a solution
λ∂x
v = A∗v − ω2v , v ∈ H
�u∗ is transversely spectrally unstable if the linear operator
λ∂x
− A∗ + ω2 is not invertible in H
u∗ is transversely spectrally unstable if the spectrum of
the linear operator λ∂x
− A∗ contains a negative value
−ω2 < 0 for some Reλ > 0.
Transverse instability problem
u∗ is transversely spectrally unstable if the spectrum
of the linear operator λ∂x
− A∗ contains a negative value
−ω2 < 0 for some Reλ > 0.
� if −ω2 is an isolated eigenvalue then u∗ is transversely
linearly unstable
� if −ω2 belongs to the essential spectrum
σess(λ∂x−A∗) = {ν ∈ C ; λ∂x−A∗−ν is not Fredholm with index 0}
then u∗ is transversely essentially unstable
Periodic waves
small periodic waves: ϕa, (x) = p
a, (ka, x)
� scaling: z = k
a, x , λ = k
a, Λ
rescaled operator
Λ∂z
− Ba, , B
a, = ∂2z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, )
with 2π-periodic coefficients
Co-periodic perturbations
Λ∂z
− Ba, , B
a, = ∂2z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, )
closed operator in H = L
2(0, 2π)
Theorem
1 the linear operator Λ∂z
− Ba, acting in L2(0, 2π) has a
simple negative eigenvalue.
2 the periodic wave ϕa, is transversely linearly unstable
with respect to co-periodic longitudinal perturbations.
Proof
Λ∂z
− Ba, , B
a, = ∂2z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, )
show that Ba, has a simple positive eigenvalue
� the operator Λ∂z
− Ba, is real
� perturbation argument: the negative eigenvalue of −Ba,
persists for small real Λ
(point) spectrum of Ba, ?
Proof
Spectrum of Ba, = ∂2
z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, )
use perturbation arguments: small a and
Proof
Spectrum of Ba, = ∂2
z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, )
use perturbation arguments: small a and
a = 0, = 0
B0,0 = ∂2z
(∂4z
+ ∂2z
), σ(B0,0) = {−n
2(n4 − n
2), n ∈ Z}
� 0 is a triple eigenvalue
� all other eigenvalues are negative
Proof
Spectrum of Ba, = ∂2
z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, )
use perturbation arguments: small a and
a = 0, = 0
B0,0 = ∂2z
(∂4z
+ ∂2z
), σ(B0,0) = {−n
2(n4 − n
2), n ∈ Z}
� 0 is a triple eigenvalue
� all other eigenvalues are negative
spectral decomposition for small a and
σ(Ba, ) = σ1(Ba, ) ∪ σ2(Ba, )
� σ1(Ba, ) ⊂ V , V neighborhood of 0
� σ2(Ba, ) ⊂ {ν ∈ C ; Re ν < −m}
Proof
Spectrum of Ba, = ∂2
z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, ) :
locate the small eigenvalues
Proof
Spectrum of Ba, = ∂2
z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, ) :
locate the small eigenvalues
a = 0
σ(B0, ) = {−n
2(k20n
4 − k
20n
2 − ), n ∈ Z}
� 0 is a triple eigenvalue
� all other eigenvalues are negative
Proof
Spectrum of Ba, = ∂2
z
(k4a, ∂
4z
+ k
2a, ∂
2z
− + p
a, ) :
locate the small eigenvalues
a = 0
σ(B0, ) = {−n
2(k20n
4 − k
20n
2 − ), n ∈ Z}
� 0 is a triple eigenvalue
� all other eigenvalues are negative
a 6= 0
� use symmetries and show that 0 is a double eigenvalue
� third eigenvalue: compute an expansion for small a, . . .
. . . , νa, = a
2
2
(1
4X2
+ O(a2 +
2)
)> 0
Consequences
implies essential transverse instability of periodic waves
with respect to localize perturbations
implies essential transverse instability of generalized
solitary waves with respect to localize perturbations
extend to Euler equations . . . ?
Zero surface tension
transverse spectral stability
� fully localized/bounded perturbations
KP-II equation
[H., Li, & Pelinovsky, 2017]
Count unstable eigenvalues
Hamiltonian structure: linear operator of the form JL
� J skew-adjoint operator
� L self-adjoint operator
Under suitable conditions:
n
u
(JL) ≤ n
s
(L)
�n
u
(JL) = number of unstable eigenvalues of JL
�n
s
(L) = number of negative eigenvalues of L
[well-known result, extensively used in stability problems . . . ]
[does not work very well for periodic waves . . . ]
An extended eigenvalue count
Hamiltonian structure: linear operator of the form JL
� J skew-adjoint operator
� L self-adjoint operator
There exists a self-adjoint operator K such that
(JL)(JK) = (JK)(JL)
Under suitable conditions:
n
u
(JL) ≤ n
s
(K)
�n
u
(JL) = number of unstable eigenvalues of JL
�n
s
(K) = number of negative eigenvalues of K
Stability of periodic waves
classical result: allows to show (orbital) stability of periodic
waves with respect to co-periodic perturbations
particular case n
s
(K) = 0: used to show nonlinear (orbital)
stability of periodic waves with respect to subharmonic
perturbations (for the KdV and NLS equations)
[Deconinck, Kapitula, 2010; Gallay, Pelinovsky, 2015]
Stability of periodic waves
classical result: allows to show (orbital) stability of periodic
waves with respect to co-periodic perturbations
particular case n
s
(K) = 0: used to show nonlinear (orbital)
stability of periodic waves with respect to subharmonic
perturbations (for the KdV and NLS equations)
[Deconinck, Kapitula, 2010; Gallay, Pelinovsky, 2015]
key step: construction of a nonnegative operator K
� relies upon the existence of a conserved higher-order energy
functional (due to integrability)
KP-II equation
Kadomtsev-Petviashivili equation
(ut
+ 6uux
+ u
xxx
)x
+ u
yy
= 0
one-parameter family of one-dimensional periodic
traveling waves (up to symmetries)
u(x , t) = φ
(x + t)
� speed > 1
� 2π-periodic, even profile φ
satisfying the KdV equation
v
′′(x) + v(x) + 3v2(x) = 0
� known explicitly!
Linearized equation
linearized KP-II equation
(wt
+ w
xxx
+ w
x
+ 6(φ
(x)w)x
)x
+ w
yy
= 0
� 2π-periodic coefficients in x
� Ansatz
w(x , y , t) = e
λt+ipy
W (x), λ ∈ C, p ∈ R
linearized equation for W (x)
λWx
+W
xxxx
+ W
xx
+ 6(φ
(x)W )xx
− p
2W = 0
Spectral stability problem
linearized equation for W (x)
λWx
+W
xxxx
+ W
xx
+ 6(φ
(x)W )xx
− p
2W = 0
the periodic wave φ
is spectrally stable iff the linear
operator
A ,p(λ) = λ∂
x
+ ∂4x
+ ∂2x
+ 6∂2x
(φ
(x) ·) − p
2
is invertible for Reλ > 0.
� 2D bounded perturbations: space Cb
(R) and p ∈ R.
� continuous spectrum . . .
Floquet/Bloch decomposition
A ,p(λ) is invertible in C
b
(R) iff the operators
A ,p(λ, γ) = λ(∂
x
+ iγ) + (∂x
+ iγ)4 + (∂x
+ iγ)2 + 6(∂x
+ iγ)2(φ
(x) ·) − p
2
are invertible in L2per
(0, 2π), for any γ ∈ [0, 1).
� γ ∈ (0, 1) : study the spectrum of the operator
B ,p(γ) = −(∂
x
+ iγ)3 − (∂x
+ iγ) − 6(∂x
+ iγ)(φ
(x) ·) + p
2(∂x
+ iγ)−1
� γ = 0 : restrict to functions with zero mean
Counting criterion
apply the counting criterion to
B ,p(γ) = J (γ)L
,p(γ)
� skew-adjoint operator J (γ) = (∂x
+ iγ)
� self-adjoint operator
L ,p(γ) = −(∂
x
+ iγ)2 − − 6φ
(x) + p
2(∂x
+ iγ)−2
construct positive commuting operators K ,p(γ)
� find commuting operators M ,p(γ)
� show that suitable linear combination of M ,p(γ) and
L ,p(γ) is a positive operator
Commuting operators
natural candidate: use a higher-order conserved functional
� resulting operator satisfies the commutativity relation
� cannot obtain positive operators . . .
Commuting operators
natural candidate: use a higher-order conserved functional
� resulting operator satisfies the commutativity relation
� cannot obtain positive operators . . .
second option: use the operators from the KdV equation
�p = 0 corresponds to the KdV equation
� decompose:
L ,p = LKdV + p
2LKP, M ,p = MKdV + p
2MKP
� MKdV is obtained from a higher order conserved functional:
MKdV = ∂4x
+ 10∂x
φ
(x)∂x
− 10 φ
(x)−
2
� compute MKP directly from the commutativity relation:
MKP =5
3
(1 + ∂−2
x
)
Main result
Transverse spectral stability of periodic waves (with
respect to bounded perturbations):
� there exist constants b such that the operators
K ,p,b(γ) = M
,p(γ)− bL ,p(γ) are positive1
� the commutativity relation holds
� the general counting criterion implies that the spectra of
B ,p(γ) = J (γ)L
,p(γ) are purely imaginary
Main result
Transverse spectral stability of periodic waves (with
respect to bounded perturbations):
� there exist constants b such that the operators
K ,p,b(γ) = M
,p(γ)− bL ,p(γ) are positive1
� the commutativity relation holds
� the general counting criterion implies that the spectra of
B ,p(γ) = J (γ)L
,p(γ) are purely imaginary
Consequence: transverse linear stability of the periodic
waves with respect to doubly periodic perturbations
Many open problems . . .
• water waves: other parameter regimes, other types of waves
(solitary waves, three-dimensional waves) . . .
• periodic waves: nonlinear stability with respect to localized
perturbations (KdV equation?) . . .
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