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Preprint submitted on 5 Sep 2021
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Error estimates of local energy regularization for the logarithmic Schrodinger equation
Weizhu Bao, Rémi Carles, Chunmei Su, Qinglin Tang
To cite this version:
Weizhu Bao, Rémi Carles, Chunmei Su, Qinglin Tang. Error estimates of local energy regularization
for the logarithmic Schrodinger equation. 2021. �hal02817115v2�
ERROR ESTIMATES OF LOCAL ENERGY REGULARIZATION FOR THE LOGARITHMIC SCHR ¨ ODINGER EQUATION
WEIZHU BAO, R´ EMI CARLES, CHUNMEI SU, AND QINGLIN TANG
Abstract. The logarithmic nonlinearity has been used in many partial diﬀer ential equations (PDEs) for modeling problems in various applications. Due to the singularity of the logarithmic function, it introduces tremendous diﬃculties in establishing mathematical theories, as well as in designing and analyzing nu merical methods for PDEs with such nonlinearity. Here we take the logarithmic Schr¨ odinger equation (LogSE) as a prototype model. Instead of regularizing f(ρ) = ln ρ in the LogSE directly and globally as being done in the literature, we propose a local energy regularization (LER) for the LogSE by ﬁrst regular izing F(ρ) = ρln ρ − ρ locally near ρ = 0
^{+}with a polynomial approximation in the energy functional of the LogSE and then obtaining an energy regular ized logarithmic Schr¨ odinger equation (ERLogSE) via energy variation. Linear convergence is established between the solutions of ERLogSE and LogSE in terms of a small regularization parameter 0 < ε ≪ 1. Moreover, the conserved energy of the ERLogSE converges to that of LogSE quadratically, which sig niﬁcantly improves the linear convergence rate of the regularization method in the literature. Error estimates are also presented for solving the ERLogSE by using LieTrotter splitting integrators. Numerical results are reported to conﬁrm our error estimates of the LER and of the timesplitting integrators for the ERLogSE. Finally our results suggest that the LER performs better than regularizing the logarithmic nonlinearity in the LogSE directly.
1. Introduction
The logarithmic nonlinearity appears in physical models from many fields. For example, the logarithmic nonlinearity is introduced in quantum mechanics or quan tum optics, where a logarithmic Schr¨odinger equation (LogSE) is considered (e.g.
[14–16, 44]),
i∂
tu = − ∆u + λ u ln  u  ^{2} , λ ∈ R ;
in oceanography and in fluid dynamics, with a logarithmic Kortewegde Vries (KdV) equation or a logarithmic KadomtsevPetviashvili (KP) equation (e.g. [39, 50, 51]);
in quantum field theory and in inflation cosmology, via a logarithmic KleinGordon equation (e.g. [12, 35, 49]); or in material sciences, by the introduction of a Cahn Hilliard (CH) equation with logarithmic potentials (e.g. [24, 28, 33]). Recently, the
2020 Mathematics Subject Classification. 22E46, 53C35, 57S235Q40, 35Q55, 65M15, 81Q05.
Key words and phrases. Logarithmic Schr¨ odinger equation; logarithmic nonlinearity; energy regularization; error estimates; convergence rate; LieTrotter splitting.
This work was partially supported by the Ministry of Education of Singapore grant R146000 296112 (MOE2019T21063) (W. Bao), Rennes M´ etropole through its AIS program (R. Carles), the Alexander von Humboldt Foundation (C. Su), the Institutional Research Fund from Sichuan University (No. 2020SCUNL110) and the National Natural Science Foundation of China (No.
11971335) (Q. Tang).
1
heat equation with a logarithmic nonlinearity has been investigated mathematically [1, 22].
In the context of quantum mechanics, the logarithmic nonlinearity was selected by assuming the separability of noninteracting subsystems property (cf. [14]). This means that a solution of the nonlinear equation for the whole system can be con structed, as in the linear theory, by taking the product of two arbitrary solutions of the nonlinear equations for the subsystems. In other words, no correlations are introduced for noninteracting subsystems. As for the physical reality, robust physical grounds have been found for the application of equations with logarithmic nonlinearity. For instance, it was found in the stochastic formulation of quantum mechanics [45, 48] that the logarithmic nonlinear term originates naturally from an internal stochastic force due to quantum fluctuations. Such kind of nonlin earity also appears naturally in inflation cosmology and in supersymmetric field theories [11, 30].
Remarkably enough for a nonlinear PDE, many explicit solutions are available for the logarithmic mechanics (see e.g. [14, 43]). For example, the logarithmic KdV equation, the logarithmic KP equation, the logarithmic KleinGordon equation give Gaussons: solitary wave solutions with Gaussian shapes [50, 51]. In the case of LogSE (see [17,31]), or the heat equation [1], every initial Gaussian function evolves as a Gaussian: solving the corresponding nonlinear PDE is equivalent to solving ordinary differential equations (involving the purely time dependent parameters of the Gaussian). However we emphasize that this is not so in the case of, e.g., the logarithmic KdV equation, the logarithmic KP equation, or the logarithmic Klein Gordon equation. This can be directly seen by trying to plug time dependent Gaussian functions into these equations. Note that this distinction between various PDEs regarding the propagation of Gaussian functions is the same as at the linear level.
The wellposedness of the Cauchy problem for logarithmic equations is not trivial since the logarithmic nonlinearity is not locally Lipschitz continuous, due to the singularity of the logarithm at the origin. Existence was proved by compactness argument based on regularization of the nonlinearity, for the CH equation with a logarithmic potential [29] and the LogSE [18]. Uniqueness is also a challenging question, settled in the case of LogSE thanks to a surprising inequality discovered in [20], recalled in Lemma 2.1 below.
The singularity of the logarithmic nonlinearity also makes it very challenging to design and analyze numerical schemes. There have been extensive numerical works for the CH equation with a logarithmic Flory Huggins energy potential [23, 25, 34, 40, 41, 52]. Specifically, a regularized energy functional was adopted for the CH equation with a logarithmic free energy [25,52]. A regularization of the logarithmic nonlinearity was introduced and analyzed in [4, 5] in the case LogSE, see also [46].
In this paper, we introduce and analyze numerical methods for logarithmic equa tions via a local energy regularization. We consider the LogSE as an example; the regularization can be extended to other logarithmic equations. The LogSE which arises in a model of nonlinear wave mechanics reads (cf. [14]),
(1.1)
( i∂
tu(x, t) = − ∆u(x, t) + λ u(x, t) f (  u(x, t)  ^{2} ), x ∈ Ω, t > 0,
u(x, 0) = u 0 (x), x ∈ Ω,
where t and x ∈ R
^{d}(d = 1, 2, 3) represent the temporal and spatial coordi nates, respectively, λ ∈ R \{ 0 } measures the force of the nonlinear interaction, u := u( x , t) ∈ C is the dimensionless wave function, and
(1.2) f (ρ) = ln ρ, ρ > 0, with ρ =  u  ^{2} .
The spatial domain is either Ω = R
^{d}, or Ω ⊂ R
^{d}bounded with Lipschitz continuous boundary; in the latter case the equation is subject to homogeneous Dirichlet or periodic boundary conditions. This model has been widely applied in quantum mechanics, nuclear physics, geophysics, open quantum systems and BoseEinstein condensation, see e.g. [3, 26, 37, 38, 53]. We choose to consider positive time only merely to simplify the presentation, since (1.1) is time reversible. Formally, the flow of (1.1) enjoys two important conservations. The mass, defined as
(1.3) N (t) := N (u( · , t)) = k u k ^{2} = Z
Ω  u(x, t)  ^{2} dx ≡ N(u 0 ), t ≥ 0, and the energy, defined as
E(t) : = E(u( · , t)) = Z
Ω
∇ u(x, t)  ^{2} dx + λF (  u(x, t)  ^{2} ) dx
≡ Z
Ω
∇ u 0 ( x )  ^{2} + λF (  u 0 ( x )  ^{2} )
d x = E(u 0 ), t ≥ 0, (1.4)
where
(1.5) F (ρ) = Z
ρ0
f (s)ds = Z
ρ0
ln s ds = ρ ln ρ − ρ, ρ ≥ 0.
The total angular momentum is also conserved, an identity that we do not use in the present paper. For the Cauchy problem (1.1) in a suitable functional framework, we refer to [17, 20, 36]. For stability properties of standing waves for (1.1), we refer to [2, 18, 21]. For the analysis of breathers and the existence of multisolitons, see [31, 32].
In order to avoid numerical blowup of the logarithmic nonlinearity at the ori gin, two models of regularized logarithmic Schr¨odinger equation (RLogSE) were proposed in [5], involving a direct regularization of f in (1.2), relying on a small regularized parameter 0 < ε ≪ 1,
(1.6)
( i∂
tu
^{ε}(x, t) = − ∆u
^{ε}(x, t) + λ u
^{ε}(x, t) f e
^{ε}(  u
^{ε}(x, t)  ) ^{2} ), x ∈ Ω, t > 0, u
^{ε}(x, 0) = u 0 (x), x ∈ Ω,
and (1.7)
( i∂
tu
^{ε}(x, t) = − ∆u
^{ε}(x, t) + λ u
^{ε}(x, t) f b
^{ε}(  u
^{ε}(x, t)  ^{2} )), x ∈ Ω, t > 0, u
^{ε}(x, 0) = u 0 (x), x ∈ Ω.
Here, f e
^{ε}(ρ) and f b
^{ε}(ρ) are two types of regularization for f (ρ), given by
(1.8) f e
^{ε}(ρ) = 2 ln(ε + √ ρ), f b
^{ε}(ρ) = ln(ε ^{2} + ρ), ρ ≥ 0, with ρ =  u
^{ε} ^{2} . Again, the RLogSEs (1.6) and (1.7) conserve the mass (1.3) with u = u
^{ε}, as well as the energies
(1.9) E e
^{ε}(t) := E e
^{ε}(u
^{ε}( · , t)) = Z
Ω
h
∇ u
^{ε}(x, t)  ^{2} dx + λ F e
^{ε}(  u
^{ε}(x, t)  ^{2} ) i
dx ≡ E e
^{ε}(u 0 ),
and (1.10)
E b
^{ε}(t) := E b
^{ε}(u
^{ε}( · , t)) = Z
Ω
h ∇ u
^{ε}(x, t)  ^{2} dx + λ F b
^{ε}(  u
^{ε}(x, t)  ^{2} ) i
dx ≡ E b
^{ε}(u 0 ), respectively, with, for ρ ≥ 0,
F e
^{ε}(ρ) = Z
ρ0
f e
^{ε}(s)ds = 2ρ ln(ε + √ ρ) + 2ε √ ρ − ρ − 2ε ^{2} ln(1 + √ ρ/ε), F b
^{ε}(ρ) =
Z
ρ0
f b
^{ε}(s)ds = (ε ^{2} + ρ) ln(ε ^{2} + ρ) − ρ − 2ε ^{2} ln ε.
(1.11)
The idea of this regularization is that the function ρ 7→ ln ρ causes no (analytical or numerical) problem for large values of ρ, but is singular at ρ = 0. A linear conver gence was established between the solutions of the LogSE (1.1) and the regularized model (1.6) or (1.7) for bounded Ω in terms of the small regularization parameter 0 < ε ≪ 1, i.e.,
sup
t∈
[0,T] k u
^{ε}(t) − u(t) k
L^{2}(Ω) = O(ε), ∀ T > 0.
Applying this regularized model, a semiimplicit finite difference method (FDM) and a timesplitting method were proposed and analyzed for the LogSE (1.6) in [5]
and [4] respectively. The above regularization saturates the nonlinearity in the region { ρ < ε ^{2} } (where ρ =  u
^{ε} ^{2} ), but of course has also some (smaller) effect in the other region { ρ > ε ^{2} } , i.e., it regularizes f (ρ) = ln ρ globally.
Energy regularization is a method which has been adapted in different fields for dealing with singularity and/or roughness: in materials science, for establishing the wellposedness of the Cauchy problem for the CH equation with a logarithmic potential [29], and for treating strongly anisotropic surface energy [7,42]; in mathe matical physics, for the wellposedness of the LogSE [18]; in scientific computing, for designing regularized numerical methods in the presence of singularities [9, 25, 52].
The main goal of this paper is to present a local energy regularization (LER) for the LogSE (1.1). We regularize the interaction energy density F(ρ) only locally in the region { ρ < ε ^{2} } by a sequence of polynomials, and keep it unchanged in { ρ > ε ^{2} } . The choice of the regularized interaction energy density F
_{n}^{ε}is prescribed by the regularity n imposed at this step, involving the matching conditions at { ρ = ε ^{2} } . We then obtain a sequence of energy regularized logarithmic Schr¨odinger equations (ERLogSEs), from the regularized energy functional density F
_{n}^{ε}, via energy varia tion. Unlike in [25, 52], where the interaction energy density F (ρ) is approximated by a second order polynomial near the origin, here we present a systematic way to regularize the interaction energy density near the origin, i.e. locally, by a se quence of polynomials such that the order of regularity n of the overall regularized interaction energy density is arbitrary. We establish convergence rates between the solutions of ERLogSEs and LogSE in terms of the small regularized parameter 0 < ε ≪ 1. In addition, we also prove error estimates of numerical approximations of ERLogSEs by using timesplitting integrators.
The rest of this paper is organized as follows. In Section 2, we introduce a
sequence of regularization F
_{n}^{ε}for the logarithmic potential. A regularized model
is derived and analyzed in Section 3 via the LER of the LogSE. Some numerical
methods are proposed and analyzed in Section 4. In Section 5, we present numerical
experiments. Throughout the paper, we adopt the standard L ^{2} based Sobolev
spaces as well as the corresponding norms, and denote by C a generic positive constant independent of ε, the time step τ and the function u, and by C(c) a generic positive constant depending on c.
2. Local regularization for F(ρ) = ρ ln ρ − ρ
We consider a local regularization starting from an approximation to the inter action energy density F (ρ) in (1.5) (and thus in (1.4)).
2.1. A sequence of local regularization. In order to make a comparison with the former global regularization (1.6), we again distinguish the regions { ρ > ε ^{2} } and { ρ < ε ^{2} } . Instead of saturating the nonlinearity in the second region, we regularize it locally as follows. For an arbitrary integer n ≥ 2, we approximate F (ρ) by a piecewise smooth function which is polynomial near the origin,
(2.1) F
_{n}^{ε}(ρ) = F(ρ)χ
_{{}ρ≥ε^{2}}+ P
_{n+1}^{ε}(ρ)χ
_{{}ρ<ε^{2}}, n ≥ 2,
where 0 < ε ≪ 1 is a small regularization parameter, χ
Ais the characteristic function of the set A, and P
_{n+1}^{ε}is a polynomial of degree n + 1. We demand F
_{n}^{ε}∈ C
^{n}([0, + ∞ )) and F
_{n}^{ε}(0) = F(0) = 0 (this allows the regularized energy to be welldefined on the whole space). The above conditions determine P
_{n+1}^{ε}, as we now check. Since P
_{n+1}^{ε}(0) = 0, write
(2.2) P
_{n+1}^{ε}(ρ) = ρ Q
^{ε}_{n}(ρ),
with Q
^{ε}_{n}a polynomial of degree n. Correspondingly, denote F (ρ) = ρ Q(ρ) with Q(ρ) = ln ρ − 1. The continuity conditions read
P
_{n+1}^{ε}(ε ^{2} ) = F(ε ^{2} ), (P
_{n+1}^{ε})
^{′}(ε ^{2} ) = F
^{′}(ε ^{2} ), . . . , (P
_{n+1}^{ε}) ^{(n)} (ε ^{2} ) = F ^{(n)} (ε ^{2} ), which in turn yield
Q
^{ε}_{n}(ε ^{2} ) = Q(ε ^{2} ), (Q
^{ε}_{n})
^{′}(ε ^{2} ) = Q
^{′}(ε ^{2} ), . . . , (Q
^{ε}_{n}) ^{(n)} (ε ^{2} ) = Q ^{(n)} (ε ^{2} ).
Thus Q
^{ε}_{n}is nothing else but Taylor polynomial of Q of degree n at ρ = ε ^{2} , i.e., (2.3) Q
^{ε}_{n}(ρ) = Q(ε ^{2} ) +
X
nk=1
Q ^{(k)} (ε ^{2} )
k! (ρ − ε ^{2} )
^{k}= ln ε ^{2} − 1 − X
nk=1
1 k
1 − ρ ε ^{2}
k.
In particular, Taylor’s formula yields (2.4) Q(ρ) − Q
^{ε}_{n}(ρ) =
Z
ρ ε^{2}Q ^{(n+1)} (s) (ρ − s)
^{n}n! ds =
Z
ρ ε^{2}(s − ρ)
^{n}s
^{n+1}ds.
Plugging (2.3) into (2.2), we get the explicit formula of P
_{n+1}^{ε}(ρ). We emphasize a formula which will be convenient for convergence results:
(2.5) (Q
^{ε}_{n})
^{′}(ρ) = 1 ε ^{2}
X
nk=1
1 − ρ ε ^{2}
k−1
= 1 ρ
1 − 1 − ρ
ε ^{2}
n, 0 ≤ ρ ≤ ε ^{2} .
2.2. Properties of the local regularization functions. Differentiating (2.1) with respect to ρ and noting (2.2), (2.3) and (2.5), we get
(2.6) f
_{n}^{ε}(ρ) = (F
_{n}^{ε})
^{′}(ρ) = ln ρ χ
_{{}ρ≥ε^{2}}+ q
_{n}^{ε}(ρ)χ
_{{}ρ<ε^{2}}, ρ ≥ 0, where
q
_{n}^{ε}(ρ) = (P
_{n+1}^{ε})
^{′}(ρ) = Q
^{ε}_{n}(ρ) + ρ (Q
^{ε}_{n})
^{′}(ρ)
= ln(ε ^{2} ) − n + 1 n
1 − ρ ε ^{2}
n−
n−
1
X
k=1
1 k
1 − ρ ε ^{2}
k.
Noticing that q
^{ε}_{n}is increasing in [0, ε ^{2} ], f e
^{ε}and f b
^{ε}are increasing on [0, ∞ ), thus all three types of regularization (2.1) and (1.11) preserve the convexity of F . Moreover, as a sequence of local regularization (or approximation) for the semi smooth function F (ρ) ∈ C ^{0} ([0, ∞ )) ∩ C
^{∞}((0, ∞ )), we have F
_{n}^{ε}∈ C
^{n}([0, + ∞ )) for n ≥ 2, while F e
^{ε}∈ C ^{1} ([0, ∞ )) ∩ C
^{∞}((0, ∞ )) and F b
^{ε}∈ C
^{∞}([0, ∞ )). Similarly, as a sequence of local regularization (or approximation) for the logarithmic function f (ρ) = ln ρ ∈ C
^{∞}((0, ∞ )), we observe that f
_{n}^{ε}∈ C
^{n}^{−}^{1} ([0, ∞ )) for n ≥ 2, while f b
^{ε}∈ C
^{∞}([0, ∞ )) and f e
^{ε}∈ C ^{0} ([0, ∞ )) ∩ C
^{∞}((0, ∞ )).
Recall the following lemma, established initially in [20, Lemma 1.1.1].
Lemma 2.1. For z 1 , z 2 ∈ C , we have Im z 1 ln(  z 1  ^{2} ) − z 2 ln(  z 2  ^{2} )
(z 1 − z 2 ) ≤ 2  z 1 − z 2  ^{2} ,
where Im(z) and z denote the imaginary part and the complex conjugate of z, re spectively.
Next we highlight some properties of f
_{n}^{ε}.
Lemma 2.2. Let n ≥ 2 and ε > 0. For z 1 , z 2 ∈ C , we have
 f
_{n}^{ε}(  z 1  ^{2} ) − f
_{n}^{ε}(  z 2  ^{2} )  ≤ 4n  z 1 − z 2  max { ε, min { z 1  ,  z 2 }} , (2.7)
Im
z 1 f
_{n}^{ε}(  z 1  ^{2} ) − z 2 f
_{n}^{ε}(  z 2  ^{2} )
(z 1 − z 2 ) ≤ 4n  z 1 − z 2  ^{2} , (2.8)
 ρ(f
_{n}^{ε})
^{′}(ρ)  ≤ 3,  √ ρ(f
_{n}^{ε})
^{′}(ρ)  ≤ 2n
ε ,  ρ ^{3/2} (f
_{n}^{ε})
^{′′}(ρ)  ≤ 3n ^{2}
2ε , ρ ≥ 0, (2.9)
 f
_{n}^{ε}(ρ)  ≤ max { ln A  , 2 + ln(nε
^{−}^{2} ) } , ρ ∈ [0, A].
(2.10)
Proof. When  z 1  ,  z 2  ≥ ε, we have f
_{n}^{ε}(  z 1  ^{2} ) − f
_{n}^{ε}(  z 2  ^{2} ) = 2 ln
1 +  z 1  −  z 2 
min { z 1  ,  z 2 }
≤ 2  z 1 − z 2  min { z 1  ,  z 2 } . A direct calculation gives
(2.11) (f
_{n}^{ε})
^{′}(ρ) = 1
ρ χ
_{{}ρ≥ε^{2}}+ n ε ^{2} 1 − ρ
ε ^{2}
n−1
+ 1 ε ^{2}
n−
1
X
k=0
1 − ρ ε ^{2}
k!
χ
_{{}ρ<ε^{2}}.
Thus when  z 1  <  z 2  ≤ ε, we have f
_{n}^{ε}(  z 1  ^{2} ) − f
_{n}^{ε}(  z 2  ^{2} ) =
Z
_{}z2^{2}z1^{2}
(f
_{n}^{ε})
^{′}(ρ)dρ
= n ε ^{2}
Z
_{}z2^{2}z1^{2}
1 − ρ ε ^{2}
n−1
dρ + 1 ε ^{2}
n
X
−1
k=0
Z
_{}z2^{2}z1^{2}
1 − ρ ε ^{2}
kdρ
≤ n
ε ^{2} (  z 2  ^{2} −  z 1  ^{2} ) + 1 ε ^{2}
n−
1
X
k=0
(  z 2  ^{2} −  z 1  ^{2} )
= 2n
ε ^{2} (  z 2  ^{2} −  z 1  ^{2} ) ≤ 4n
ε  z 1 − z 2  .
Another case when  z 2  <  z 1  ≤ ε can be established similarly. Supposing, for example,  z 2  < ε <  z 1  , denote by z 3 the intersection point of the circle { z ∈ C :
 z  = ε } and the line segment connecting z 1 and z 2 . Combining the inequalities above, we have
f
_{n}^{ε}(  z 1  ^{2} ) − f
_{n}^{ε}(  z 2  ^{2} ) ≤  f
_{n}^{ε}(  z 2  ^{2} ) − f
_{n}^{ε}(  z 3  ^{2} )  +  ln(  z 1  ^{2} ) − ln(  z 3  ^{2} ) 
≤ 4n
ε  z 2 − z 3  + 2
ε  z 1 − z 3 
≤ 4n
ε (  z 2 − z 3  +  z 1 − z 3  ) = 4n
ε  z 1 − z 2  , which completes the proof for (2.7).
Noticing that Im
z 1 f
_{n}^{ε}(  z 1  ^{2} ) − z 2 f
_{n}^{ε}(  z 2  ^{2} )
(z 1 − z 2 )
= − Im(z 1 z 2 )f
_{n}^{ε}(  z 2  ^{2} ) − Im(z 1 z 2 )f
_{n}^{ε}(  z 1  ^{2} )
= Im(z 1 z 2 )
f
_{n}^{ε}(  z 1  ^{2} ) − f
_{n}^{ε}(  z 2  ^{2} )
= 1
2i (z 1 z 2 − z 1 z 2 )
f
_{n}^{ε}(  z 1  ^{2} ) − f
_{n}^{ε}(  z 2  ^{2} ) , and
 z 1 z 2 − z 1 z 2  =  z 2 (z 1 − z 2 ) + z 2 (z 2 − z 1 )  ≤ 2  z 2   z 1 − z 2  ,
 z 1 z 2 − z 1 z 2  =  z 1 (z 2 − z 1 ) + z 1 (z 1 − z 2 )  ≤ 2  z 1   z 1 − z 2  , which implies
 z 1 z 2 − z 1 z 2  ≤ 2 min { z 1  ,  z 2 }  z 1 − z 2  , one can conclude (2.8) by applying (2.7).
It follows from (2.11) that
g(ρ) = ρ(f
_{n}^{ε})
^{′}(ρ) = χ
_{{}ρ≥ε^{2}}+ nρ ε ^{2} 1 − ρ
ε ^{2}
n−1
+ ρ ε ^{2}
n
X
−1
k=0
1 − ρ ε ^{2}
k!
χ
_{{}ρ<ε^{2}}= χ
_{{}ρ≥ε^{2}}+ nρ ε ^{2} 1 − ρ
ε ^{2}
n−1
+ 1 − 1 − ρ ε ^{2}
nχ
_{{}ρ<ε^{2}}, which gives that
g
^{′}(ρ)χ
_{{}ρ<ε^{2}}= n ε ^{2} 1 − ρ
ε ^{2}
n−2
2 − (n + 1)ρ ε ^{2}
.
This leads to
 ρ(f
_{n}^{ε})
^{′}(ρ)  = g(ρ) ≤ max { 1, g 2ε ^{2}
n + 1
} ≤ 1 + 2n n + 1 ≤ 3,
which completes the proof for the first inequality in (2.9). Finally it follows from (2.11) that
√ ρ(f
_{n}^{ε})
^{′}(ρ) = 1
√ ρ χ
_{{}ρ≥ε^{2}}+
√ ρ
ε ^{2} n 1 − ρ ε ^{2}
n−1
+
n−
1
X
k=0
1 − ρ ε ^{2}
k!
χ
_{{}ρ<ε^{2}},
(f
_{n}^{ε})
^{′′}(ρ) = − 1
ρ ^{2} χ
_{{}ρ≥ε^{2}}− n ^{2} − 1 ε ^{4} 1 − ρ
ε ^{2}
n−2
+ 1 ε ^{4}
n
X
−3
k=0
(k + 1) 1 − ρ ε ^{2}
k!
χ
_{{}ρ<ε^{2}},
which immediately yields that
 √ ρ(f
_{n}^{ε})
^{′}(ρ)  ≤ 2n ε ,
 ρ ^{3/2} (f
_{n}^{ε})
^{′′}(ρ)  ≤ 1
ε n ^{2} − 1 +
n
X
−3
k=0
(k + 1)
!
= 3n(n − 1) 2ε < 3n ^{2}
2ε . For ρ ∈ [0, ε ^{2} ], in view of ε ∈ (0, 1], one deduces
 f
_{n}^{ε}(ρ)  ≤ ln(ε
^{−}^{2} ) + n + 1 n
1 − ρ
ε ^{2}
n+
n
X
−1
k=1
1 k
1 − ρ
ε ^{2}
k≤ ln(ε
^{−}^{2} ) + n + 1
n +
n−
1
X
k=1
1 k
≤ ln(ε
^{−}^{2} ) + 2 + X
nk=2
1 k
≤ 2 + ln(nε
^{−}^{2} ),
which together with  f
_{n}^{ε}(ρ)  ≤ max { ln(ε
^{−}^{2} ),  ln(A) } when ρ ∈ [ε ^{2} , A] concludes
(2.10).
2.3. Comparison between different regularizations. To compare different reg ularizations for F(ρ) (and thus for f (ρ)), Fig. 1 shows F
_{n}^{ε}(n = 2, 4, 100, 500), F e
^{ε}and F b
^{ε}for different ε, from which we can see that the newly proposed local regu larization F
_{n}^{ε}approximates F more accurately.
Fig. 2 shows various regularizations f
_{n}^{ε}(n = 2, 4, 100, 500), f e
^{ε}and f b
^{ε}for various ε, while Figs. 3 & 4 show their first and secondorder derivatives. From these figures, we can see that the newly proposed local regularization f
_{n}^{ε}(and its deriva tives with larger n) approximates the nonlinearity f (and its derivatives) more accurately. In addition, Fig. 5 depicts F
_{n}^{ε}(ρ) (with ε = 0.1) and its derivatives for different n, from which we can clearly see the convergence of F
_{n}^{ε}(ρ) (and its derivatives) to F(ρ) (and its derivatives) W.R.T. order n.
3. Local energy regularization (LER) for the LogNLS In this section, we consider the regularized energy
(3.1) E
_{n}^{ε}(u) :=
Z
Ω
∇ u  ^{2} + λF
_{n}^{ε}(  u  ^{2} ) dx,
where F
_{n}^{ε}is defined in (2.1). The Hamiltonian flow of the regularized energy i∂
tu =
δE_{n}^{ε}
(u)
δu
yields the following energy regularized logarithmic Schr¨odinger equation (ERLogSE) with a regularizing parameter 0 < ε ≪ 1,
(3.2)
( i∂
tu
^{ε}(x, t) = − ∆u
^{ε}(x, t) + λ u
^{ε}(x, t) f
_{n}^{ε}(  u
^{ε}(x, t)  ^{2} ), x ∈ Ω, t > 0, u
^{ε}(x, 0) = u 0 (x), x ∈ Ω.
We recall that f
_{n}^{ε}is defined by (2.6).
Figure 1. Comparison of different regularizations for F (ρ) = ρ ln ρ − ρ.
3.1. The Cauchy problem. To investigate the wellposedness of the problem (3.2), we first introduce some appropriate spaces. For α > 0 and Ω = R
^{d}, denote by L ^{2}
_{α}the weighted L ^{2} space
L ^{2}
_{α}:= { v ∈ L ^{2} ( R
^{d}), x 7−→ h x i
^{α}v( x ) ∈ L ^{2} ( R
^{d}) } , where h x i := p
1 +  x  ^{2} , with norm k v k
L^{2}_{α}:= kh x i
^{α}v( x ) k
L^{2}(
R^{d}) . Regarding the Cauchy problem (3.2), we have similar results as for the regularization (1.6) in [5], but not quite the same. For the convenience of the reader, we recall the main arguments.
Theorem 3.1. Let λ ∈ R , u 0 ∈ H ^{1} (Ω), and 0 < ε ≤ 1.
(1). For (3.2) posed on Ω = R
^{d}or a bounded domain Ω with homogeneous Dirichlet
or periodic boundary condition, there exists a unique, global weak solution u
^{ε}∈
Figure 2. Comparison of different regularizations for the nonlin earity f (ρ) = ln ρ.
L
^{∞}_{loc} ( R ; H ^{1} (Ω)) to (3.2) (with H _{0} ^{1} (Ω) instead of H ^{1} (Ω) in the Dirichlet case). Fur thermore, for any given T > 0, there exists a positive constant C(λ, T ) (independent of n) such that
(3.3) k u
^{ε}k
L^{∞}([0,T ];H
^{1}(Ω)) ≤ C(λ, T ) k u 0 k
H^{1}(Ω) , ∀ ε > 0.
(2). For (3.2) posed on a bounded domain Ω with homogeneous Dirichlet or periodic boundary condition, if in addition u 0 ∈ H ^{2} (Ω), then u
^{ε}∈ L
^{∞}_{loc} ( R ; H ^{2} (Ω)) and there exists a positive constant C(n, λ, T ) such that
(3.4) k u
^{ε}k
L^{∞}([0,T];H
^{2}(Ω)) ≤ C(n, λ, T ) k u 0 k
H^{2}(Ω) , ∀ ε > 0.
(3). For (3.2) on Ω = R
^{d}, suppose moreover u 0 ∈ L ^{2}
_{α}, for some 0 < α ≤ 1.
Figure 3. Comparison of different regularizations for f
^{′}(ρ) = 1/ρ.
• There exists a unique, global weak solution u
^{ε}∈ L
^{∞}_{loc} ( R ; H ^{1} ( R
^{d}) ∩ L ^{2}
_{α}) to (3.2), and
k u
^{ε}k
L^{∞}([0,T];H
^{1}) ≤ C(n, λ, T ) k u 0 k
H^{1},
k u
^{ε}k
L^{∞}([0,T];L
^{2}_{α}) ≤ C(n, λ, T, k u 0 k
H^{1}) k u 0 k
L^{2}_{α}, ∀ ε > 0.
(3.5)
• If in addition u 0 ∈ H ^{2} ( R
^{d}), then u
^{ε}∈ L
^{∞}_{loc} ( R ; H ^{2} ( R
^{d})), and (3.6) k u
^{ε}k
L^{∞}([0,T];H
^{2}) ≤ C(n, λ, T, k u 0 k
H^{2}, k u 0 k
L^{2}_{α}), ∀ ε > 0.
• If u 0 ∈ H ^{2} ( R
^{d}) ∩ L ^{2} _{2} , then u
^{ε}∈ L
^{∞}_{loc} ( R ; H ^{2} ( R
^{d}) ∩ L ^{2} _{2} ).
Proof. (1). For fixed ε > 0, the nonlinearity in (3.2) is locally Lipschitz contin
uous, and grows more slowly than any power of  u
^{ε} . Standard Cauchy theory
for nonlinear Schr¨odinger equations implies that there exists a unique solution
u
^{ε}∈ L
^{∞}_{loc} ( R ; H ^{1} (Ω)) to (3.2) (respectively, u
^{ε}∈ L
^{∞}_{loc} ( R ; H _{0} ^{1} (Ω)) in the Dirichlet
case); see e.g. [19, Corollary 3.3.11 and Theorem 3.4.1]. In addition, the L ^{2} norm
Figure 4. Comparison of different regularizations for f
^{′′}(ρ) =
− 1/ρ ^{2} .
of u
^{ε}is independent of time,
k u
^{ε}(t) k ^{2}
L^{2}(Ω) = k u 0 k ^{2}
L^{2}(Ω) , ∀ t ∈ R . For j ∈ { 1, . . . , d } , differentiate (3.2) with respect to x
j:
(i∂
t+ ∆) ∂
ju
^{ε}= λ∂
ju
^{ε}f
_{n}^{ε}(  u
^{ε} ^{2} ) + 2λu
^{ε}(f
_{n}^{ε})
^{′}(  u
^{ε} ^{2} )Re (u
^{ε}∂
ju
^{ε}) .
Multiply the above equation by ∂
ju
^{ε}, integrate on Ω, and take the imaginary part:
(2.9) implies
1 2
d
dt k ∂
ju
^{ε}k ^{2}
L^{2}(Ω) ≤ 6  λ k ∂
ju
^{ε}k ^{2}
L^{2}(Ω) , hence (3.3), by Gronwall lemma.
(2). The propagation of the H ^{2} regularity is standard, since f
_{n}^{ε}is smooth, so we
focus on (3.4). We now differentiate (3.2) with respect to time: we get the same
Figure 5. Comparison of regularizations g
_{n}^{0.1} (g = F, f, f
^{′}, f
^{′′}) with different order n.
estimate as above, with ∂
jreplaced by ∂
t, and so
k ∂
tu
^{ε}(t) k ^{2}
L^{2}(Ω) ≤ k ∂
tu
^{ε}(0) k ^{2}
L^{2}(Ω) e ^{12}
^{}^{λ}^{}^{t}^{}. In view of (3.2),
i∂
tu
^{ε}_{}_{t=0}= − ∆u 0 + λu 0 f
_{n}^{ε}(  u 0  ^{2} ).
For 0 < δ < 1, we have
√ ρ  f
_{n}^{ε}(ρ)  ≤ C(δ)
ρ ^{1/2}
^{−}^{δ/2}+ ρ ^{1/2+δ/2} ,
for some C(δ) independent of ε and n, so for δ > 0 sufficiently small, Sobolev embedding entails
k ∂
tu
^{ε}(0) k
L^{2}(Ω) ≤ k u 0 k
H^{2}(Ω) + C(δ)
k u 0 k ^{1}
_{L}^{−}^{2−2δ}^{δ}_{(Ω)} + k u 0 k ^{1+δ}
H^{1}(Ω)
. Since Ω is bounded, H¨ older inequality yields
k u 0 k
L^{2−2δ}(Ω) ≤ k u 0 k
L^{2}(Ω  Ω 
^{δ/(2}^{−}^{2δ)} .
Thus, the first term in (3.2) is controlled in L ^{2} . Using the same estimates as above, we control the last term in (3.2) (thanks to (3.3)), and we infer an L ^{2} estimate for
∆u
^{ε}, hence (3.4).
(3). In the case Ω = R
^{d}, we multiply (3.2) by h x i
^{α}, and the same energy estimate as before now yields
d
dt k u
^{ε}k ^{2}
L^{2}_{α}= 4α Im Z
R^{d}
x · ∇ u
^{ε}h x i ^{2}
^{−}^{2α} u
^{ε}(t) d x . k h x i ^{2α}
^{−}^{1} u
^{ε}k
L^{2}(
R^{d}) k∇ u
^{ε}k
L^{2}(
R^{d})
. k h x i
^{α}u
^{ε}k
L^{2}(R
^{d}) k∇ u
^{ε}k
L^{2}(R
^{d}) ,
where the last inequality follows from the assumption α ≤ 1, hence (3.5). To prove (3.6), we resume the same approach as to get (3.4), with the difference that the H¨ older estimate must be replaced by some other estimate (see e.g. [17]): for δ > 0 sufficiently small,
Z
R^{d}
 u  ^{2}
^{−}^{2δ} . k u k ^{2}
_{L}^{−}^{2}_{(R} ^{2δ}
^{d}^{−}_{)}
^{dδ/α}k x 
^{α}u k
^{dδ/α}_{L}^{2}_{(R}
^{d}_{)} .
The L ^{2} _{2} estimate follows easily, see e.g. [5] for details.
3.2. Convergence of the regularized model. In this subsection, we show an approximation property of the regularized model (3.2) to (1.1).
Lemma 3.2. Suppose the equation (3.2) is set on Ω, where Ω = R ^{d} , or Ω ⊂ R ^{d} is a bounded domain with homogeneous Dirichlet or periodic boundary condition. We have the general estimate:
(3.7) d
dt k u ^{ε} (t) − u(t) k ^{2} L
^{2}≤  λ  4 k u ^{ε} (t) − u(t) k ^{2} L
^{2}+ 6ε k u ^{ε} (t) − u(t) k L
^{1}. Proof. Subtracting (1.1) from (3.2), we see that the error function e ^{ε} := u ^{ε} − u satisfies
i∂ t e ^{ε} + ∆e ^{ε} = λ
u ^{ε} ln(  u ^{ε}  ^{2} ) − u ln(  u  ^{2} ) + λu ^{ε}
f _{n} ^{ε} (  u ^{ε}  ^{2} ) − ln(  u ^{ε}  ^{2} )
χ
_{{}u
^{ε}<ε
}. Multiplying the above error equation by e ^{ε} (t), integrating in space and taking imaginary parts, we can get by using Lemma 2.1, (2.4) and (2.5) that
1 2
d
dt k e ^{ε} (t) k ^{2} L
^{2}= 2λ Im Z
Ω
[u ^{ε} ln(  u ^{ε}  ) − u ln(  u  )] e ^{ε} (x, t)dx + λ Im
Z

u
^{ε}<ε
u ^{ε}
f _{n} ^{ε} (  u ^{ε}  ^{2} ) − ln(  u ^{ε}  ^{2} )
e ^{ε} (x, t)dx
≤ 2  λ k e ^{ε} (t) k ^{2} L
^{2}+  λ  Z

u
^{ε}<ε
u ^{ε} e ^{ε}
Q ^{ε} _{n} (  u ^{ε}  ^{2} ) − ln(  u ^{ε}  ^{2} ) +  u ^{ε}  ^{2} (Q ^{ε} _{n} )
^{′}(  u ^{ε}  ^{2} ) dx
≤ 2  λ k e ^{ε} (t) k ^{2} L
^{2}+  λ  Z

u
^{ε}<ε
e ^{ε} u ^{ε} h Z ^{ε}
^{2}
u
^{ε}^{2}(s −  u ^{ε}  ^{2} ) ^{n}
s ^{n+1} ds − 1 +  u ^{ε}  ^{2} (Q ^{ε} _{n} )
^{′}(  u ^{ε}  ^{2} ) i dx
= 2  λ  k e ^{ε} (t) k ^{2} L
^{2}+  λ  Z

u
^{ε}<ε
e ^{ε} u ^{ε} h Z ^{ε}
^{2}
u
^{ε}^{2}(s −  u ^{ε}  ^{2} ) ^{n} s ^{n+1} ds −
1 −  u ^{ε}  ^{2} ε ^{2}
n i dx
≤ 2  λ  k e ^{ε} (t) k ^{2} L
^{2}+ ε  λ k e ^{ε} k L
^{1}+  λ  Z ε
^{2}0
s
^{−}^{n}
^{−}^{1} Z

u
^{ε}^{2}<s
e ^{ε} u ^{ε} (s −  u ^{ε}  ^{2} ) ^{n} dxds
≤ 2  λ  k e ^{ε} (t) k ^{2} L
^{2}+ 3ε  λ k e ^{ε} k L
^{1}.
This yields the result.
Invoking the same arguments as in [5], based on the previous error estimate, and interpolation between L ^{2} and H ^{2} , we get the following error estimate.
Proposition 3.3. If Ω has finite measure and u 0 ∈ H ^{2} (Ω), then for any T > 0,
k u ^{ε} − u k L
^{∞}([0,T];L
^{2}(Ω)) ≤ C 1 ε, k u ^{ε} − u k L
^{∞}([0,T];H
^{1}(Ω)) ≤ C 2 ε ^{1/2} ,
where C 1 depends on  λ  , T ,  Ω  , and C 2 depends in addition on k u 0 k H
^{2}(Ω) . If Ω = R ^{d} , 1 ≤ d ≤ 3 and u 0 ∈ H ^{2} ( R ^{d} ) ∩ L ^{2} _{2} , then for any T > 0, we have
k u ^{ε} − u k L
^{∞}([0,T];L
^{2}(
R^{d})) ≤ D 1 ε
^{4+d}^{4}, k u ^{ε} − u k L
^{∞}([0,T];H
^{1}(
R^{d})) ≤ D 2 ε
^{4+d}^{2}, where D 1 and D 2 depend on d,  λ  , T , k u 0 k L
^{2}_{2}and k u 0 k H
^{2}(
R^{d}) .
Proof. The proof is the same as that in [5]. We just list the outline for the readers’
convenience. When Ω is bounded, the convergence in L ^{2} follows from Gronwall’s inequality by applying (3.7) and the estimate k v k L
^{1}≤  Ω  ^{1/2} k v k L
^{2}. The estimate in H ^{1} follows form the GagliardoNirenberg inequality k v k H
^{1}≤ C k v k ^{1/2} L
^{2}k v k ^{1/2} H
^{2}and the property (3.4). For Ω = R ^{d} , the convergence in L ^{2} can be established by Gronwall’s inequality and the estimate (cf. [5])
k v k L
^{1}≤ C d k v k ^{1} L
^{−}^{2}^{d/4} k v k ^{d/4} _{L}
^{2}_{2}≤ C d
ε
^{−}^{1} k v k ^{2} L
^{2}+ ε
^{4−d}^{4+d}k v k
2d 4+d
L
^{2}_{2},
which is derived by the CauchySchwarz inequality and Young’s inequality. The convergence in H ^{1} can similarly derived by the GagliardoNirenberg inequality.
3.3. Convergence of the energy. By construction, the energy is conserved, i.e., (3.8) E _{n} ^{ε} (u ^{ε} ) =
Z
Ω
∇ u ^{ε} (x, t)  ^{2} + λF _{n} ^{ε} (  u ^{ε} (x, t)  ^{2} )
dx = E _{n} ^{ε} (u 0 ).
For the convergence of the energy, we have the following estimate.
Proposition 3.4. For u 0 ∈ H ^{1} (Ω) ∩ L ^{α} (Ω) with α ∈ (0, 2), the energy E _{n} ^{ε} (u 0 ) converges to E(u 0 ) with
 E _{n} ^{ε} (u 0 ) − E(u 0 )  ≤  λ  k u 0 k ^{α} L
^{α}ε ^{2}
^{−}^{α} 1 − α/2 . In addition, for bounded Ω, we have
 E _{n} ^{ε} (u 0 ) − E(u 0 )  ≤  λ   Ω  ε ^{2} .
Proof. It can be deduced from the definition (3.8) and (2.4) that
 E _{n} ^{ε} (u 0 ) − E(u 0 )  =  λ  Z
Ω
[F (  u 0 (x)  ^{2} ) − F _{n} ^{ε} (  u 0 (x)  ^{2} )]dx
=  λ  Z

u
0(
x)
<ε  u 0 (x)  ^{2} [Q(  u 0 (x)  ^{2} ) − Q ^{ε} _{n} (  u 0 (x)  ^{2} )]dx
=  λ  Z

u
0(
x)
<ε  u 0 (x)  ^{2} Z ε
^{2}
u
0(
x)
^{2}s
^{−}^{n}
^{−}^{1} (s −  u 0 (x)  ^{2} ) ^{n} dsdx
=  λ  Z ε
^{2}0
s
^{−}^{n}
^{−}^{1} Z
