Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics
We construct identities of Pohozhaev type, in the context of elastostatics and elastodynamics, by using the Noetherian approach. As an application, a non-existence result for forced semi-linear isotropic and anisotropic elastic systems is established.
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irk-123456789-1471762019-02-14T01:24:46Z Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics Bozhkov, Y. Olver, P.J. We construct identities of Pohozhaev type, in the context of elastostatics and elastodynamics, by using the Noetherian approach. As an application, a non-existence result for forced semi-linear isotropic and anisotropic elastic systems is established. 2011 Article Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics / Y. Bozhkov, P.J. Olver // Symmetry, Integrability and Geometry: Methods and Applications. — 2011. — Т. 7. — Бібліогр.: 31 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 35J50; 35J47; 35L51 DOI:10.3842/SIGMA.2011.055 http://dspace.nbuv.gov.ua/handle/123456789/147176 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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We construct identities of Pohozhaev type, in the context of elastostatics and elastodynamics, by using the Noetherian approach. As an application, a non-existence result for forced semi-linear isotropic and anisotropic elastic systems is established. |
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Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics |
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Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics |
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Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics |
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Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics |
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Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics / Y. Bozhkov, P.J. Olver // Symmetry, Integrability and Geometry: Methods and Applications. — 2011. — Т. 7. — Бібліогр.: 31 назв. — англ. |
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Symmetry, Integrability and Geometry: Methods and Applications |
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AT bozhkovy pohozhaevandmorawetzidentitiesinelastostaticsandelastodynamics AT olverpj pohozhaevandmorawetzidentitiesinelastostaticsandelastodynamics |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 7 (2011), 055, 9 pages
Pohozhaev and Morawetz Identities
in Elastostatics and Elastodynamics?
Yuri BOZHKOV † and Peter J. OLVER ‡
† Instituto de Matemática, Estatistica e Computação Cient́ıfica - IMECC,
Universidade Estadual de Campinas - UNICAMP, Rua Sérgio Buarque de Holanda, 651,
13083-859 - Campinas - SP, Brasil
E-mail: bozhkov@ime.unicamp.br
URL: http://www.ime.unicamp.br/~bozhkov/
‡ School of Mathematics, University of Minnesota, Minneapolis, MN 55455, USA
E-mail: olver@umn.edu
URL: http://www.math.umn.edu/~olver/
Received February 01, 2011, in final form June 02, 2011; Published online June 08, 2011
doi:10.3842/SIGMA.2011.055
Abstract. We construct identities of Pohozhaev type, in the context of elastostatics and
elastodynamics, by using the Noetherian approach. As an application, a non-existence result
for forced semi-linear isotropic and anisotropic elastic systems is established.
Key words: Pohozhaev identity; Navier’s equations; Noether’s theorem
2010 Mathematics Subject Classification: 35J50; 35J47; 35L51
1 Introduction
Identities of Pohozhaev type have been widely used in the theory of partial differential equations,
in particular for establishing non-existence results for large classes of forced elliptic boundary
value problems and eigenvalue problems, [20, 21, 22]. The purpose of this note is to obtain and
apply analogous identities in elastostatics and elastodynamics, which have not (to the authors’
knowledge) been developed to date. Our approach will be based on a fundamental identity
first introduced by Noether in her seminal paper [16] that connected symmetries of variational
problems to conservation laws of their Euler–Lagrange equations.
As noted in [19], the identities originally due to Pohozhaev, [20, 21], owe their existence to
Noether’s identity. For classical solutions of the linear equation ∆u+λu = 0 such an identity was
obtained by Rellich in [24]. Further, in [25], Rellich established an integral identity for a function
belonging to certain function spaces, without any reference to differential equations it may
satisfy. The Rellich identity has been generalized by Mitidieri, [12, 13], for a pair of functions.
General Rellich-type identities on Riemannian manifolds have been recently established in [5, 6]
by use of Noether’s identity applied to conformal Killing vector fields.
In [20], Pohozhaev established an integral identity for solutions of the Dirichlet problem
for the semilinear Poisson equation ∆u + λf(u) = 0 in a bounded domain with homogeneous
Dirichlet boundary condition. Later, for solutions of general Dirichlet problems, he obtained
in [21] what is now called the Pohozhaev identity. Such identities became very popular after
the paper of Pucci and Serrin, [22], where, on p. 683, the relation with the general Noetherian
theory is mentioned. See also the earlier paper by Knops and Stuart, [11], and remarks in the
second author’s 1986 book [19]. The Noetherian approach to Pohozhaev’s identities was further
?This paper is a contribution to the Special Issue “Symmetry, Separation, Super-integrability and Special
Functions (S4)”. The full collection is available at http://www.emis.de/journals/SIGMA/S4.html
mailto:bozhkov@ime.unicamp.br
http://www.ime.unicamp.br/~bozhkov/
mailto:olver@umn.edu
http://www.math.umn.edu/~olver/
http://dx.doi.org/10.3842/SIGMA.2011.055
http://www.emis.de/journals/SIGMA/S4.html
2 Y. Bozhkov and P.J. Olver
developed and applied in [3, 4, 23, 31]. Additional applications of Rellich–Pohozhaev estimates
to nonlinear elliptic theory can be found in [26, 27], while applications to nonlocal problems
appear in [8]. With regard to geometric applications, [1, 7, 9, 28] develop a systematic approach
to Pohozhaev-type obstructions for partial differential equations invariant under the action of
a conformal group. For a relation between the Lie point symmetries of the nonlinear Poisson
equation on a (pseudo-) Riemannian manifold and its isometry and conformal groups see [2].
In dynamical problems, the conformal invariance of the wave and Klein–Gordon equations
was used by Morawetz, [14], to establish several very useful integral identities. These were
applied by her and Strauss, [29, 30], to the study of the decay, stability, and scattering of waves
in nonlinear media. In the final section, we will generalize Morawetz’ conformal identity to some
dynamical systems governing waves in elastic media. Applications of our identity to decay and
scattering of elastic waves will be treated elsewhere.
In elastostatics, the independent variables x ∈ Rn, for n ≥ 2, represent reference body coor-
dinates, while the dependent variables u = u(x) = (u1(x), . . . , un(x)) represent the deformation
of the point x. The independent variable x will belong to a bounded or unbounded domain
Ω ⊆ Rn that has sufficiently (piecewise) smooth boundary ∂Ω. We use ν to denote the outward
unit normal on ∂Ω. For elastodynamics, we append an additional independent variable, t, repre-
senting the time, and so u = u(t, x). The partial derivatives of a smooth (vector) function u(x)
are denoted by subscripts:
uki :=
∂uk
∂xi
, ukt :=
∂uk
∂t
, ukij :=
∂2uk
∂xi∂xj
, etc.
The n× n spatial Jacobian matrix ∇u = (uki ) is known as the deformation gradient.
We shall consistently use the Einstein summation convention over repeated indices, which
always run from 1 to n. We assume that all considered functions, vector fields, tensors, functio-
nals, etc. are sufficiently smooth in order that all the derivatives we write exist in the classical
sense. When we say that a function is “arbitrary”, we mean that it is a sufficiently smooth
function of its arguments defined on the domain Ω. Extensions of our results to more general
solutions will then proceed on a case by case basis.
2 Noether’s identity
A vector field
v = ξi(x, u)
∂
∂xi
+ φi(x, u)
∂
∂ui
on the space of independent and dependent variables induces a flow that can be interpreted as
a (local) one-parameter group of transformations. The vector field is known as the infinitesimal
generator of the flow, [19]. For example, the particular vector field
v = axi
∂
∂xi
+ b ui
∂
∂ui
,
where a, b are constant, generates the group of scaling transformations
(x, u) 7−→
(
λax, λbu
)
.
The action of the group on functions u = f(x) by transforming their graphs induces an action
on their derivatives. The corresponding infinitesimal generator of the prolonged group action
has the form
pr(1)v = ξi(x, u)
∂
∂xi
+ φi(x, u)
∂
∂ui
+ φij(x, u,∇u)
∂
∂uij
, (1)
Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics 3
where
φij(x, u,∇u) = Djφ
i − (Djξ
k)uik =
∂φi
∂xj
+
∂φi
∂uk
ukj −
∂ξk
∂xj
uik −
∂ξk
∂ul
ulju
i
k, (2)
and Dj = ∂/∂xj + ukj∂/∂u
k denotes the total derivative with respect to xj . See [19] for a proof
of this formula, along with its extension to higher order derivatives.
For a first order Lagrangian L(x, u,∇u), Noether’s identity reads
pr(1)v(L) + LDiξ
i = Ei(L)(φi − uijξj) +Di
[
Lξi +
∂L
∂uji
(φj − ujsξs)
]
, (3)
where Ei is the Euler operator or variational derivative with respect to ui, [19]. Once stated,
the verification of the identity is a straightforward computation. In the following sections, we
will investigate how to use Noether’s identity in the framework of elasticity, and apply the
corresponding integral identities to establish non-existence results. The proofs are sketched,
while the full details are left to the interested reader as exercises.
3 Elastostatics
We recall that the equilibrium equations for a homogeneous isotropic linearly elastic medium in
the absence of body forces arise from the variational principle with Lagrangian
L0(x, u,∇u) =
1
2
µ‖∇u ‖2 +
1
2
(µ+ λ)(∇ · u)2 =
1
2
µ
n∑
i,j=1
(
uij
)2
+
1
2
(µ+ λ)
(
n∑
i=1
uii
)2
,
where the parameters λ and µ are the Lamé moduli. The squared norm of the deformation
gradient matrix ∇u refers to the sum of the squares of its entries, while ∇ · u denotes the
divergence of the deformation. The corresponding Euler–Lagrange equations are known as
Navier’s equations:
µ∆u+ (µ+ λ)∇(∇ · u) = 0,
where the Laplacian ∆ acts component-wise on u. Henceforth, we assume that µ > 0 and
µ+λ > 0, thereby ensuring strong ellipticity and positive definiteness of the underlying elasticity
tensor, [10, 17].
In this paper, we shall study boundary value problems for elastic bodies that are subject to
a nonlinear body-force potential F (u). Thus, we modify the preceding Lagrangian
L(x, u,∇u) =
1
2
µ‖∇u ‖2 +
1
2
(µ+ λ)(∇ · u)2 − F (u),
where we assume, without loss of generality, that F (0) = 0. The associated equilibrium Euler–
Lagrange equations are
µ∆u+ (µ+ λ)∇(∇ · u) + f(u) = 0, (4)
where fi(u) = ∂F/∂ui are the components of the gradient of the body-force potential with
respect to the dependent variables u.
More generally, we consider Lagrangians of the form:
L =
1
2
Ckl
ij e
i
ke
j
l − F (u), (5)
4 Y. Bozhkov and P.J. Olver
where again F (0) = 0, and
e =
1
2
(
∇u+∇uT
)
, with components eik =
1
2
(
uik + uki
)
,
is the strain tensor. The quadratic components in the Lagrangian (5) model the stored energy
of a general anisotropic linearly elastic medium, while F (u) represents a nonlinear body-force
potential. The elastic moduli Ckl
ij are assumed to be constant, satisfying
Ckl
ij = Cil
kj = Ckj
il = C lk
ji . (6)
Thus in planar elasticity there are 6 independent elastic moduli, while in three dimensions 21
independent moduli are required in general, [10]. Additional symmetry restrictions stemming
from the constitutive properties of the elastic material may place additional constraints on the
moduli. We may also assume
Ckl
ij a
i
ka
j
l ≥ 0 (7)
for any matrix A = (apq). The less restrictive Legendre–Hadamard condition is that
Ckl
ij v
ivjwkwl > 0
for any rank one matrix A = v ⊗ w. The Euler–Lagrange equations associated with (5) read
Ckl
ij u
j
kl + fi(u) = 0. (8)
In general, the most basic Pohozhaev-type identity is based on the associated Noether identity
for the infinitesimal generator of an adroitly chosen scaling transformation group, [19].
Theorem 1. Let Ω be a bounded domain in Rn. Then the classical solutions of (8) – that is
u ∈ C2(Ω) ∩ C1(Ω̄) – subject to homogeneous Dirichlet boundary conditions on ∂Ω satisfy the
following Pohozhaev-type identity:∫
Ω
[
n− 2
2
ukfk(u)− nF (u)
]
dx = −1
2
∫
∂Ω
Ckl
ij u
i
ku
j
l (x, ν)ds, (9)
where ν is the outward unit normal to ∂Ω and (·, ·) is the Euclidean scalar product in Rn.
Proof. We consider the one-parameter group of dilations
(x, u) 7−→ (λx, λ(2−n)/2u)
with infinitesimal generator
v = xi
∂
∂xi
+
2− n
2
ui
∂
∂ui
.
According to (1), (2), the first order prolongation of this vector field is
pr(1)v = xi
∂
∂xi
+
2− n
2
ui
∂
∂ui
− n
2
uji
∂
∂uji
.
Then one easily sees that
pr(1)v(L) + LDiξ
i =
n− 2
2
ukfk(u)− nF (u). (10)
The identity (9) now follows from the divergence theorem using (3), (5), (10), our assumption
F (0) = 0, and the homogeneous Dirichlet boundary conditions, taking into account that, on ∂Ω,
ujsνi = ujiνs. (11)
See [22, p. 683] for more details on the last point. �
Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics 5
For the sake of completeness, we specialize the general elastic Pohozhaev identity to the
isotropic case of the forced Navier equations (4) in Ω:∫
Ω
[
n− 2
2
ukfk(u)− nF (u)
]
dx = −1
2
∫
∂Ω
[
1
2
µ‖∇u ‖2 +
1
2
(µ+ λ)(∇ · u)2
]
(x, ν)ds,
again subject to homogeneous Dirichlet boundary conditions on ∂Ω.
As a corollary, we obtain the following non-existence result. Recall that the domain Ω is
star-shaped with respect to the origin if (x, ν) ≥ 0 for any x ∈ ∂Ω.
Theorem 2. Suppose that Ω is a star-shaped domain. Let the function
F = F (s) = F (s1, . . . , sn) ∈ C1(Rn)
satisfy the conditions F (0) = 0 and
n− 2
2
sk
∂F
∂sk
− nF (s) ≥ 0, i = 1, . . . , n, (12)
for any s ∈ Rn. We also suppose that the equality in (12) holds if and only if s = 0. Then
there is no non-trivial classical solution of the potential systems (4), (8), subject to homogeneous
Dirichlet boundary conditions.
Proof. This theorem follows easily from the identity (9), taking into account the positivity
requirement (7) and star-shapedness condition. Indeed, any classical solution of (8) subject to
homogeneous Dirichlet boundary conditions on ∂Ω must satisfy the identity (9). For (7) and the
star-shapedness condition (x, ν) ≥ 0 for any x ∈ ∂Ω, it follows that the right-hand side of (9) is
non-positive. On the other hand, by (12) the left-hand side of (9) is positive unless u = 0 in Ω.
Hence u = 0. �
4 Elastodynamics
In this section, we turn our attention to hyperbolic elastodynamic systems of potential type:
−uitt + Ckl
ij u
j
kl + fi(u) = 0 (13)
in R × Ω with homogeneous Dirichlet boundary conditions on R × ∂Ω. The corresponding
Lagrangian is given by
L =
1
2
Ckl
ij e
i
ke
j
l −
1
2
uitu
i
t − F (u) =
1
2
Ckl
ij u
i
ku
j
l −
1
2
uitu
i
t − F (u), (14)
where the second expression follows from the requirements (6) on the elastic moduli.
Theorem 3. The classical solutions of the problem (13) satisfy the following identity
d
dt
∫
Ω
[
tE(u) + uitu
i
kx
k +
n− 1
2
uiuit
]
dx =
∫
Ω
[
n− 1
2
ukfk(u)− (n+ 1)F (u)
]
dx
+
∫
∂Ω
[
1
2
(
Ckl
ij u
i
ku
j
l +
1
2
uitu
i
t
)
(x, ν) + t Ckl
ij u
i
ku
j
l
]
ds, (15)
where
E(u) =
1
2
(
Ckl
ij e
i
ke
j
l + uitu
i
t
)
− F (u) =
1
2
(
Ckl
ij u
i
ku
j
l + uitu
i
t
)
− F (u)
is the energy density.
6 Y. Bozhkov and P.J. Olver
Proof. We introduce a vector field v which is the infinitesimal generator of the dilation group
(t, x, u) 7−→
(
λt, λx, λ(1−n)/2u
)
.
The first order prolongation of v is given by
pr(1)v = t
∂
∂t
+ xi
∂
∂xi
+
1− n
2
ui
∂
∂ui
− n+ 1
2
uit
∂
∂uit
− n+ 1
2
uij
∂
∂uij
.
As a result,
pr(1)v(L) + LDiξ
i =
n− 1
2
ukfk(u)− (n+ 1)F (u), (16)
where the Lagrangian L is given by (14). Then, after some algebraic manipulations, the identi-
ty (15) follows from the Noether identity (3) combined with (14), (11), (16), the homogeneous
Dirichlet boundary conditions, and, finally, the divergence theorem. �
Let Ω ⊂ Rn be a ball of radius R centered at the origin. If we assume that u(t, x) decays
sufficiently rapidly as R = |x| → ∞, then the following conformal identity holds for the nonlinear
hyperbolic system (13) in R× Rn:
Corollary 1. The classical solutions of the problem (13) in R× Rn that decay rapidly at large
distances satisfy the identity
d
dt
∫
Rn
[
tE(u) + uitu
i
kx
k +
n− 1
2
uiuit
]
dx =
∫
Rn
[
n− 1
2
ukfk(u)− (n+ 1)F (u)
]
dx.
We observe that this result generalizes Morawetz’s dilational identity for nonlinear wave
equations, [14, 29, 30], to elastodynamical systems.
Finally, we consider a nonlinear hyperbolic system of so-called Hamiltonian type, [4],
−uitt + Ckl
ij u
j
kl +Hvi = 0,
−vitt + Ckl
ij v
j
kl +Hui = 0, (17)
in R×Ω with homogeneous Dirichlet boundary conditions on R×∂Ω. (The independent variab-
le xmust belong to an even dimensional space R2m.) For such systems, we obtain a generalization
of Morawetz’s conformal identity [30].
Theorem 4. The classical solutions of the problem (17) satisfy the following identity
d
dt
∫
Ω
[
tE(u, v) +
(
xkujkv
j
t + xkvjku
j
t
)
+
n− 1
2
(
aujvjt + bvjujt
)]
dx
=
∫
Ω
[
n− 1
2
(
aukHuk + bvkHvk
)]
dx
+
∫
∂Ω
[(
Ckl
ij u
i
kv
j
l + uitv
i
t
)
(x, ν) + tCkl
ij
(
uitv
j
l νk + vjtu
i
kνl
)]
ds, (18)
where the constants a and b are such that a+ b = 2 and
E(u, v) = Ckl
ij u
i
kv
j
l + uitv
i
t −H(u, v).
Pohozhaev and Morawetz Identities in Elastostatics and Elastodynamics 7
Proof. In order to prove Theorem 4, we use the same scheme as in the preceding Theorem 3.
Namely, we consider a vector field v which is the infinitesimal generator of the dilation group
(t, x, u, v) 7−→
(
λt, λx, λa(1−n)/2u, λb(1−n)/2v
)
,
where the constants a and b satisfy a+ b = 2. Applying the first order prolongation
pr(1)v = t
∂
∂t
+ xi
∂
∂xi
+
a(1− n)
2
ui
∂
∂ui
+
b(1− n)
2
vi
∂
∂vi
+
(
a(1− n)
2
− 1
)
uit
∂
∂uit
+
(
a(1− n)
2
− 1
)
uij
∂
∂uij
+
(
b(1− n)
2
− 1
)
vit
∂
∂vit
+
(
b(1− n)
2
− 1
)
vij
∂
∂vij
to the Lagrangian
L =
1
2
Ckl
ij u
i
kv
j
l − u
i
tv
i
t −H(u, v) (19)
yields
pr(1)v(L) + LDiξ
i =
n− 1
2
(
aukHuk + bvkHvk
)
, (20)
when a+ b = 2. Then, after some additional work, the identity (18) follows from (19), (3), (20),
(11), the homogeneous Dirichlet boundary conditions, and the divergence theorem. �
Corollary 2. Let a, b and E be as in Theorem 4. Then, provided u and v decay sufficiently
rapidly at large distances,
d
dt
∫
R2m
[
tE(u, v) +
(
xkujkv
j
t + xkvjku
j
t
)
+
n− 1
2
(
aujvjt + bvjujt
)]
dx
=
∫
R2m
[
n− 1
2
(
aukHuk + bvkHvk
)]
dx.
Applications of these identities to the stability and scattering of waves in elastic media will
be developed elsewhere.
5 Further directions
We emphasize that, in order to obtain Pohozhaev and Morawetz-type identities in elastostatics
and elastodynamics by the Noetherian approach developed in [3, 4], we have focussed our atten-
tion on dilations, which are particular cases of conformal transformations. Further variational
identities associated with other variational symmetries remain to be investigated. In particular,
it would be interesting to analyze the variational identity for the semilinear Navier equations
that corresponds to the first order generalized symmetry
v =
[
µuij + (2µ+ λ)δiju
k
k
] ∂
∂uj
found in [18]. In fact, the variational and (at least in three dimensions) non-variational symme-
tries for isotropic linear elastostatics were completely classified in [17, 18] and the systems not
only admit point symmetries, but also a number of first order generalized symmetries. In the
two-dimensional case, complex variable methods, as in [15], are used to produce infinite families
of symmetries and conservation laws. Also in the two-dimensional case, additional symmetries
appear when 3µ+ λ = 0. In the three-dimensional case, when 7µ+ 3λ = 0, Navier’s equations
admit a full conformal symmetry group, along with additional conformal-like generalized sym-
metries. Although these restrictions are non-physical, they still lead to interesting divergence
identities in the more general isotropic case, which can be applied to the analysis of eigenvalue
problems, and also, potentially, the nonlinearly forced case. This remains to be investigated
thoroughly.
8 Y. Bozhkov and P.J. Olver
Acknowledgements
We wish to thank the referees for their useful suggestions. Yuri Bozhkov would also like to thank
FAPESP and CNPq, Brasil, for partial financial support. Peter Olver was supported in part
by NSF Grant DMS 08–07317. We both would like to thank FAPESP, São Paulo, Brasil, for
the grant giving Peter Olver the opportunity to visit IMECC-UNICAMP, where this work was
initiated.
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Introduction
Noether's identity
Elastostatics
Elastodynamics
Further directions
References
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