Abstract
We study stability times for a family of parameter dependent nonlinear Schrödinger equations on the circle, close to the origin. Imposing a suitable Diophantine condition (first introduced by Bourgain), we prove a rather flexible Birkhoff Normal Form theorem, which implies, e.g., exponential and sub-exponential time estimates in the Sobolev and Gevrey class respectively.
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Notes
Namely g is a holomorphic function on the domain \(\mathbb {T}_a := {\left\{ x\in \mathbb {C}/2\pi \mathbb {Z}\, :\, {\left| {\text {Im}}x\right| }< a\right\} }\) with \(L^2\)-trace on the boundary.
A vector \(\omega \in \mathbb {R}^n\) is called diophantine when it is badly approximated by rationals, i.e. it satisfies, for some \(\gamma ,\tau >0\), \({\left| k\cdot \omega \right| } \ge \gamma {\left| k\right| }^{-\tau },\quad \forall k\in \mathbb {Z}^n{\setminus }{\left\{ 0\right\} }\,\).
Actually \({\mathtt {H}}_{p,s,a}\) also depends on \(\theta \), however, since we think \(\theta \) fixed, we omit to write explicitly the dependence on it.
Endowed with the scalar product \((u,v)_{{\mathtt {h}}_{\mathtt {w}}}:=\sum _{j\in \mathbb {Z}} \mathtt {w}_j^2 u_j {\bar{v}}_j.\)
As usual given a vector \(k\in \mathbb {Z}^\mathbb {Z}\), \(|k|:=\sum _{j\in \mathbb {Z}}|k_j|\).
Since \(\omega \in {\mathtt {D}_{\gamma ,{q}}}\), w.l.o.g. we may assume that R is in the range of the operator \(\{\sum _{j\in \mathbb {Z}} \omega _j |u_j|^2 ,\cdot \}\).
As usual \({\mathtt {w}}\le {\mathtt {w}}'\) means that \({\mathtt {w}}_j\le {\mathtt {w}}'_j\) for every \(j\in \mathbb {Z}.\)
In the sense that there exists a symplectic change of variables \(\Phi : B_{r}({\mathtt {h}}_{{\mathtt {w}}})\mapsto B_{2r}({\mathtt {h}}_{{\mathtt {w}}})\) such that \( \Psi \circ \Phi u=\Phi \circ \Psi u= u\), \(\forall u\in B_{\frac{7}{8} r}({\mathtt {h}}_{\mathtt {w}})\).
We recall that given a complex Hilbert space H with a Hermitian product \((\cdot ,\cdot )\), its realification is a real symplectic Hilbert space with scalar product and symplectic form given by
$$\begin{aligned} \langle u,v\rangle = 2\mathrm{Re}(u,v),\quad \omega (u,v)= 2\mathrm{Im}(u,v). \end{aligned}$$Denoting by \(\mu \) the measure in \(\Omega _{q}\) and by \(\nu \) the product measure on \([-1/2,1/2]^\mathbb {Z}\), then \(\mu (A)= \nu (\omega ^{(-1)}(A))\) for all sets \(A\subset \Omega _{q}\) such that \(\omega ^{(-1)}(A)\) is \(\nu \)-measurable.
Explicitely \(\Pi _{{{\mathcal {K}}}}H:=\sum _{{\varvec{{\alpha }}}={\varvec{{\beta }}}} H_{{\varvec{{\alpha }}},{\varvec{{\beta }}}}{u^{\varvec{{\alpha }}}}{\bar{u}^{\varvec{{\beta }}}},\)\(\Pi _{{{\mathcal {R}}}}H:=\sum _{{\varvec{{\alpha }}}\ne {\varvec{{\beta }}}} H_{{\varvec{{\alpha }}},{\varvec{{\beta }}}}{u^{\varvec{{\alpha }}}}{\bar{u}^{\varvec{{\beta }}}}.\)
K is the constant in (4.8).
C is defined in (3.3).
We are just using the fact that the ratio \(c^{(j)}_{r,\eta ,{\mathtt {w}}}({\varvec{{\alpha }}},{\varvec{{\beta }}}) / c^{(j)}_{r',\eta ',{\mathtt {w}}'}({\varvec{{\alpha }}},{\varvec{{\beta }}})\) depends on \(r,r'\) only through their ratio.
Note that on the preserving momentum subspace \({{\mathcal {H}}}_{r,\eta }({\mathtt {h}}_{\mathtt {w}})\) coincides with \({{\mathcal {H}}}_{r,0}({\mathtt {h}}_{\mathtt {w}})\) for every \(\eta .\)
R is defined in (1.2) and the constants in “Appendix A”.
i.e. P preserves momentum and we are assuming (1.3).
Using that \((a+b)^{\tau _1}\le 2^{{\tau _1}-1}(a^{\tau _1}+b^{\tau _1})\) for \(a,b\ge 0,\)\({\tau _1}\ge 1.\)
\([\cdot ]\) is the integer part.
Note that \(1\le p-{\mathtt {N}}\tau _\mathtt {S}< 1+\tau _\mathtt {S}.\)
Note that \(1\le p-{\mathtt {N}}\tau _\mathtt {M}< 1+\tau _\mathtt {M}.\)
We still denote it by \(u_0\).
We still denote it by \(u_0\).
Note that the function
$$\begin{aligned} y \mapsto y-3 \left( 1+ \frac{1}{6} \frac{y}{\ln (y)}\right) \left( \ln y+\ln (1+ \frac{1}{6} \frac{y}{\ln (y)} )\right) \end{aligned}$$is positive for \(y\ge 40.\)
Regarding \({C_{\mathtt {Nem}}}\) note that
$$\begin{aligned} \sup _{x\ge 1} x^p e^{- t x+s x^\theta } \le \exp {\left( (1-\theta ){\left( \frac{s}{t^\theta }\right) }^{\frac{1}{1-\theta }}\right) }\max \left\{ \frac{p }{e (1-\theta )t}, e^{-\frac{t(1-\theta )}{p}}\right\} ^p. \end{aligned}$$Namely the solution of the equation \(\partial _t \Phi (u,t)=X(\Phi (u,t))\) with initial datum \(\Phi (u,0)=u.\)
We assume \(t_0\) positive, the negative case is analogous.
The case \(T_0<0\) is analogous.
Note that the term \(\left( \frac{1}{i}+\frac{1}{(j-i)}\right) ^q\) for \(j=4\) and \(i=2\) is 1 for every q.
\(\sum _{i\ge 2} 1^{-q}\le 2^{-q}+\int _2^\infty x^{-q}dx\).
Using that for \(x,y\ge 0\) and \(0\le c\le 1\) we get \((x+y)^c\le x^c+y^c.\)
Use that \(\ln (x+y)\le \ln x+\ln y\) if \(x,y\ge 2.\)
Recall footnote 34.
Using that \(\ln (1+y)\le 1+\ln y\) for every \(y\ge 1.\)
Using that, for every fixed \(0<{\mathfrak {C}} \le 1,\) we have \({\mathfrak {C}} x\ge \ln x\) for every \(x\ge \frac{2}{{\mathfrak {C}}}\ln \frac{1}{{\mathfrak {C}}} .\)
Assume, e.g. that \(\ell _s\ne 0\), then \(|\partial _{\xi _s}\omega \cdot \ell |\ge s^{-{q}}.\)
References
Bambusi, D.: Nekhoroshev theorem for small amplitude solutions in nonlinear Schrödinger equations. Math. Z. 230(2), 345–387 (1999)
Bambusi, D.: On long time stability in Hamiltonian perturbations of nonresonant linear PDE’s. Nonlinearity 12, 823–850 (1999)
Bambusi, D.: Birkhoff normal form for some nonlinear PDEs. Commun. Math. Phys. 234(2), 253–285 (2003)
Berti, M., Biasco, L., Procesi, M.: KAM theory for the Hamiltonian derivative wave equation. Annales Scientifiques de l’ENS 46(2), 299–371 (2013)
Berti, M., Delort, J.M.: Almost Global Existence of Solutions for Capillarity–Gravity Water Waves Equations with Periodic Spatial Boundary Conditions. Springer, Berlin (2018)
Biasco, L., Di Gregorio, L.: A Birkhoff-Lewis type theorem for the nonlinear wave equation. Arch. Ration. Mech. Anal. 196(1), 303–362 (2010)
Bambusi, D., Delort, J.-M., Grébert, B., Szeftel, J.: Almost global existence for Hamiltonian semilinear Klein-Gordon equations with small Cauchy data on Zoll manifolds. Commun. Pure Appl. Math. 60(11), 1665–1690 (2007)
Benettin, G., Fröhlich, J., Giorgilli, A.: A Nekhoroshev-type theorem for Hamiltonian systems with infinitely many degrees of freedom. Commun. Math. Phys. 119(1), 95–108 (1988)
Bernier, J., Faou, E., Grébert, B.: Rational normal forms and stability of small solutions to nonlinear schrödinger equations (2018). arXiv:1812.11414
Bounemoura, A., Fayad, B., Niederman, L.: Double exponential stability for generic real-analytic elliptic equilibrium points (2015). Preprint ArXiv : arXiv.org/abs/1509.00285
Bambusi, D., Grébert, B.: Forme normale pour NLS en dimension quelconque. C. R. Math. Acad. Sci. Paris 337(6), 409–414 (2003)
Bambusi, D., Grébert, B.: Birkhoff normal form for partial differential equations with tame modulus. Duke Math. J. 135(3), 507–567 (2006)
Biasco, L., Massetti, J.E., Procesi, M.: Exponential and sub-exponential stability times for the NLS on the circle. Rendiconti Lincei - Matematica e Applicazioni 30(2), 351–364 (2019). https://doi.org/10.4171/RLM/850
Bourgain, J.: Construction of approximative and almost periodic solutions of perturbed linear Schrödinger and wave equations. Geom. Funct. Anal. 6(2), 201–230 (1996)
Bourgain, J.: On the growth in time of higher Sobolev norms of smooth solutions of Hamiltonian PDE. Int. Math. Res. Not. 6, 277–304 (1996)
Bourgain, J.: On invariant tori of full dimension for 1D periodic NLS. J. Funct. Anal. 229(1), 62–94 (2005)
Colliander, J., Keel, M., Staffilani, G., Takaoka, H., Tao, T.: Transfer of energy to high frequencies in the cubic defocusing nonlinear Schrödinger equation. Invent. Math. 181(1), 39–113 (2010)
Cong, H., Liu, J., Shi, Y., Yuan, X.: The stability of full dimensional kam tori for nonlinear Schrödinger equation. preprint (2017). arXiv:1705.01658
Cong, H., Mi, L., Wang, P.: A Nekhoroshev type theorem for the derivative nonlinear Schrödinger equation. preprint (2018)
Delort, J.M.: A quasi-linear birkhoff normal forms method. Application to the quasi-linear Klein–Gordon equation on \(\mathtt S^1\). Astérisque, 341 (2012)
Delort, J.-M., Szeftel, J.: Long-time existence for small data nonlinear Klein–Gordon equations on tori and spheres. Int. Math. Res. Not. 37, 1897–1966 (2004)
Delort, J.-M., Szeftel, J.: Bounded almost global solutions for non Hamiltonian semi-linear Klein–Gordon equations with radial data on compact revolution hypersurfaces. Ann. Inst. Fourier (Grenoble) 56(5), 1419–1456 (2006)
Faou, E., Grébert, B.: A Nekhoroshev-type theorem for the nonlinear Schrödinger equation on the torus. Anal. PDE 6(6), 1243–1262 (2013)
Faou, E., Gauckler, L., Lubich, C.: Sobolev stability of plane wave solutions to the cubic nonlinear Schrödinger equation on a torus. Commun. Partial Differ. Equ. 38(7), 1123–1140 (2013)
Feola, R., Iandoli, F.: Long time existence for fully nonlinear NLS with small Cauchy data on the circle. Ann. Sc. Norm. Super. Pisa Cl. Sci. https://doi.org/10.2422/2036-2145.201811_003
Guardia, M., Haus, E., Procesi, M.: Growth of Sobolev norms for the analytic NLS on \({\mathbb{T}}^2\). Adv. Math. 301, 615–692 (2016)
Guardia, M., Kaloshin, V.: Growth of Sobolev norms in the cubic defocusing nonlinear Schrödinger equation. J. Eur. Math. Soc. (JEMS) 17(1), 71–149 (2015)
Grébert, B., Thomann, L.: Resonant dynamics for the quintic nonlinear Schrödinger equation. Ann. Inst. H. Poincaré Anal. Non Linéaire 29(3), 455–477 (2012)
Guardia, M.: Growth of Sobolev norms in the cubic nonlinear Schrödinger equation with a convolution potential. Commun. Math. Phys. 329(1), 405–434 (2014)
Hani, Z.: Long-time instability and unbounded Sobolev orbits for some periodic nonlinear Schrödinger equations. Arch. Ration. Mech. Anal. 211(3), 929–964 (2014)
Haus, E., Procesi, M.: KAM for beating solutions of the quintic NLS. Commun. Math. Phys. 354(3), 1101–1132 (2017)
Kuksin, S., Perelman, G.: Vey theorem in infinite dimensions and its application to KDV. Discrete Contin. Dyn. Syst. 27, 1–24 (2010)
Morbidelli, A., Giorgilli, A.: Superexponential stability of KAM tori. J. Stat. Phys. 78(5–6), 1607–1617 (1995)
Mi, L., Sun, Y., Wang, P.: Long time stability of plane wave solutions to the cubic NLS on torus (2018). preprint
Nikolenko, N.V.: The method of Poincaré normal forms in problems of integrability of equations of evolution type. Russ. Math. Surv. 41(5), 63–114 (1986)
Yuan, X., Zhang, J.: Long time stability of Hamiltonian partial differential equations. SIAM J. Math. Anal. 46(5), 3176–3222 (2014)
Acknowledgements
The three authors have been supported by the ERC grant HamPDEs under FP7 n. 306414 and the PRIN Variational Methods in Analysis, Geometry and Physics. J.E. Massetti also acknowledges Centro di Ricerca Matematica Ennio de Giorgi and UniCredit Bank R&D group for financial support through the “Dynamics and Information Theory Institute” at the Scuola Normale Superiore. The authors would like to thank D. Bambusi, M. Berti, B. Grebert, Z. Hani and A. Maspero for helpful suggestions and fruitful discussions.
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Appendices
Part 3. Appendices
Appendix A. Constants
In this subsection are listed all the constants appearing along the paper. We first introduce some auxiliary constants. Given \(t,{\sigma },\zeta >0,\)\(p>1/2,\)\(0<\theta <1,\)\(s,{q}\ge 0,\) we setFootnote 26
Here are the constants appearing in Theorem 1.1:
Here are the constants appearing in Proposition 1.1
Here are the constants in Theorem 1.2
where, recalling 8.2,
Here are the constants appearing in Corollary 1.1:
Appendix B. Proofs of the Main Properties of the Norms
Lemma B.1
Let \(0<r_1<r.\) Let E be a Banach space endowed with the norm \(|\cdot |_E\). Let \(X:B_r \rightarrow E\) a vector field satisfying
Then the flow \(\Phi (u,t)\) of the vector fieldFootnote 27 is well defined for every
and \(u\in B_{r_1}\) with estimate
Proof
Fix \(u\in B_{r_1}\). Let us first prove that \(\Phi (u,t)\) exists \(\forall \, |t|\le T.\) Otherwise there exists a timeFootnote 28\(0<t_0<T\) such that \(|\Phi (u,t)|_E<r\) for every \(0\le t<t_0\) but \(|\Phi (u,t_0)|_E=r.\) Then, by the fundamental theorem of calculus
Therefore
which is a contradiction. Finally, for every \(|t|\le T,\)
\(\square \)
Proof of Lemma 2.1
For brevity we set, for every \(r'>0\)
We use Lemma B.1, with \(E\rightarrow {\mathtt {h}}_{\mathtt {w}}\), \(X\rightarrow X_S\), \(\delta _0\rightarrow (r+\rho ) |S|_{r+\rho },\)\(r\rightarrow r+\rho ,\)\(r_1\rightarrow r,\)\(T\rightarrow 8e.\) Then the fact that the time 1-Hamiltonian flow \(\Phi ^1_S: B_r({\mathtt {h}}_{\mathtt {w}}) \rightarrow B_{r + \rho }({\mathtt {h}}_{\mathtt {w}})\) is well defined, analytic, symplectic follows, since for any \(\eta \ge 0\)
Regarding the estimate (2.3), again by Lemma B.1 (choosing \(t=1\)), we get
Estimates (2.4), (2.5), (2.6) directly follow by (2.7) with \(h=0,1,2,\) respectively and \(c_k=1/k!\), recalling that by Lie series
where \( H^{(i)} := \mathrm{ad}_S^i (H)= \mathrm{ad}_S ( H^{(i-1)}) \), \( H^{(0)}:=H \).
Let us prove (2.7). Fix \(k\in {\mathbb {N}},\)\(k>0\) and set
Note that, by the immersion properties of the norm (recall Remark 2.1)
Noting that
by using k times (2.1) we have
Then, using \( k^k\le e^k k!, \) we get
Finally, if S and H satisfy mass conservation so does each \( \mathrm{ad}_S^k H \), \( k \ge 1 \), hence \( H \circ \Phi ^1_S \) too. \(\square \)
Proof of Lemma 3.1
We first prove (i). It is easily seen that:
Now
hence, in evaluating the supremum of \(|X_{\underline{H}_\eta }|_{\mathtt {w}}\) over \(|u|_{\mathtt {w}}\le r\) we ca restrict to the case in which \(u=(u_j)_{j\in \mathbb {Z}}\) has all real positive components. Hence
Then
where
since, by the reality condition 1.23, we have
By the linear map
the ball of radius 1 in \(\ell ^2\) is isomorphic to the the ball of radius r in \({\mathtt {h}}_{\mathtt {w}}\), namely \(L_{r,{\mathtt {w}}}(B_1(\ell ^2))=B_r({\mathtt {h}}_{\mathtt {w}}).\) We have
Then (i) follows.
In order to prove item (ii) we rely on the fact that, since we are using the \(\eta \)-majorant norm, the supremum over y in the norm is achieved on the real positive cone. Moreover, given \(u,v\in \ell ^2\), if
then \(|u|_{\ell ^2}\le |v|_{\ell ^2}\). \(\square \)
Proof of Lemma 5.4
Let us look at the time evolution of \(|v(t)|_{\mathtt {w}}^2\). By construction and Cauchy-Schwarz inequality
as long as \(|v(t)|_{\mathtt {w}}\le r\); namely
as long as \(|v(t)|_{\mathtt {w}}\le r.\)
Assume by contradiction that there exists a timeFootnote 29
such that
Then
By (B.5) we get
which contradicts (B.6), proving (5.26). \(\square \)
Proof of Lemma 5.5
We first note that (see, e.g. Lemma 17 of [BDG10]) for \(p >1/2\) and every sequence \(\{x_i\}_{i\in \mathbb {Z}}\), \(x_i\ge 0,\)
with \(c:=4^p\sum _{i\in \mathbb {Z}} \langle i \rangle ^{-2p}=({C_{\mathtt {alg}}(p)})^2.\) Then
Regarding the second estimate, we set
Note that
We claim that
Indeed by (B.7) we can consider only the case \(j\ge 0.\) Since \( \phi (-|i|,j)\le \phi (|i|,j) \) we can consider only the case \(i\ge 0\). Again by (B.7) we can assume \(j\ge i.\) In particular we can take \(j>i>0,\) (B.8) being trivial in the cases \(j=i,\)\(i=0\). We have
Then it remains also to discuss the case \(j-2\ge i\ge 2;\) we have
proving (B.8).
For \(q\ge 0\) set
We claim that
Indeed, since \(\lfloor j \rfloor /\lfloor j+1 \rfloor \le 1\) and \(\lfloor j \rfloor /\lfloor j-1 \rfloor \le 3/2\) for \(j\ge 0\), we haveFootnote 30
using that \((x+y)^q\le 2^{q-1}(x^q+y^q)\) for \(x,y\ge 0\) and thatFootnote 31
Note that for every \(q,q_0\ge 0\) we have
since
We now note that for \(p >1/2\), \(j\in \mathbb Z\) and every sequence \(\{x_i\}_{i\in \mathbb {Z}}\), \(x_i\ge 0,\) we have by Cauchy-Schwarz inequality
with \(c_{2p}\) defined in (B.9). Using the above inequality we get
The proof ends recalling (B.10). \(\square \)
Lemma B.2
(Nemitskii operators). Let \(p> 1/2.\) (i) Fix \(s\ge 0, a_0\ge 0\). Consider a sequence \(F^{(d)}={\left( F^{(d)}_j\right) }_{j\in \mathbb {Z}}\in {\mathtt {h}}_{p,s,a_0}\), \(d\ge 1,\) such that
for some \(R>0\).
For \(u={\left( u_j\right) }_{j\in \mathbb {Z}}\) let \({\bar{u}}= {\left( \overline{u_{-j}}\right) }_{j\in \mathbb {Z}}\) and consider the Hamiltonian
For all \((\eta ,a,r)\) such that \(\eta +a \le a_0\) and \(({C_{\mathtt {alg}}(p)}r)^2 \le R\), we have that \(H\in {{\mathcal {H}}}_{r,\eta }({\mathtt {h}}_{p,s,a})\) and
(ii) Analogously if \(F^{(d)}\) are constants satisfying
and \(({C_{\mathtt {alg},\mathtt {M}}(p)}r)^2\le R,\) then \(H\in {{\mathcal {H}}}^{r,p}\) with
Proof
(i) By definition the \(\eta \)-majorant Hamiltonian is
where
hence
consequently
Moreover
Since
we get
Therefore
(ii) The proof is analogous to point (i). \(\square \)
Proof of Proposition 6.2
We start by Taylor expanding H in homogeneous components. The majorant analiticity implies that for a homogeneous component of degree d one has
Now let us consider the polinomial map (homogeneous of degree \(d-1\)) \(X_{H^{(d)}}: {\mathtt {h}}_{p,s,a} \rightarrow {\mathtt {h}}_{p,s,a}\); as is habitual we identify the polynomial map with the corresponding symmetric multilinear operator \(M^{(d-1)}: {\mathtt {h}}_{p,s,a}^{d-1} \rightarrow {\mathtt {h}}_{p,s,a}\). Since we are in a Hilbert space, one has that
for all \(\eta \ge 0\). Now let us compute the tame norm on a homogeneous component, i.e.
where
now setting \(\pi = \sum _i j_i- j \) we have
which means that for any \(|u|_{ p_0,s,a}\le r-\rho \) one has
We conclude that
and the thesis follows since the right hand side is convergent. \(\square \)
Appendix C. Small Divisor Estimates
Let us start with two techincal lemmata.
Lemma C.1
For \(p,\beta >0\) and \(x_0\ge 0\) we have that
Lemma C.2
Let \(0<a<1\) and \(x_1\ge x_2\ge \cdots \ge x_N\ge 2.\) Then
Proof
By induction over N. It is obviously true for \(N=1.\) Assume that it hols for N and prove it for \(N+1.\)\(\square \)
Proof of Lemma 6.1
The fact that this (6.5) holds true when \(\pi =0\) is proven in [Bou96b] and [CLSY]. The bound (6.5) is equivalent to proving
i.e.
Inequality (C.2) then follows from
which we are now going to prove. We shall show that the function f(x) is increasing in \(x\ge 0\); then the result follows by showing \(f(0) \ge 0\), which is what was proven by Yuan and Bourgain.
We now verify that \(f'(x)\ge 0\). By direct computation we see that
so it suffices to prove that
which is indeed true, since \(\sum _{i\ge 2}\widehat{n}_i\ge \widehat{n}_2\ge 1\) holds, by mass conservation. \(\square \)
Proof of Lemma 7.1
In this subsection we will take
Given \(u\in \mathbb {Z}^\mathbb {Z}\), with \(|u|<\infty ,\) consider the set
where \(D<\infty \) is its cardinality. Define the vector \(m=m(u)\) as the reordering of the elements of the set above such that \(|m_1|\ge |m_2|\ge \dots \ge |m_D|\ge 1.\) Given \({\varvec{{\alpha }}}\ne {\varvec{{\beta }}}\in {\mathbb {N}}^\mathbb {Z},\) with \(|{\varvec{{\alpha }}}|=|{\varvec{{\beta }}}|<\infty \) we consider \(m=m({\varvec{{\alpha }}}-{\varvec{{\beta }}})\) and \(\widehat{n}=\widehat{n}({\varvec{{\alpha }}}+{\varvec{{\beta }}}).\) If we denote by D the cardinality of m and N the one of \(\widehat{n}\) we have
and
Set
For every function g defined on \(\mathbb {Z}\) we have that
Lemma C.3
Assume that g defined on \(\mathbb {Z}\) is non negative, even and not decreasing on \({\mathbb {N}}.\) Then, if \({\varvec{{\alpha }}}\ne {\varvec{{\beta }}}\),
Proof
By (C.8)
and (C.9) follows by (C.6) and (C.7). \(\square \)
We denote as before the momentum by \(\pi \) so by (C.8)
and
Analogously
Finally note that
Note that
indeed, by mass conservation, \(|{\varvec{{\alpha }}}|=|{\varvec{{\beta }}}|=1\) therefore if \(N=2\) we get \({\varvec{{\alpha }}}-{\varvec{{\beta }}}= e_{j_1}-e_{j_2}\) so if \(\pi =0\) we have \({\varvec{{\alpha }}}={\varvec{{\beta }}}\). Note also that
indeed, if \(D=0\) then \({\varvec{{\alpha }}}_l-{\varvec{{\beta }}}_l=0\) for every \(|l|\ge 1\) and, by mass conservation \({\varvec{{\alpha }}}_0={\varvec{{\beta }}}_0\), contradicting \({\varvec{{\alpha }}}\ne {\varvec{{\beta }}}\) .
Lemma C.4
Given \({\varvec{{\alpha }}}\ne {\varvec{{\beta }}}\in {\mathbb {N}}^\mathbb {Z},\) with \(1\le |{\varvec{{\alpha }}}|=|{\varvec{{\beta }}}|<\infty \) and satisfying (7.1), we haveFootnote 32
Proof
In the case \(D=1\) by (C.10) \(|\pi |=|m_1|\) and (C.16) follows. Let us now consider the case \(D=2\), i.e.
Let us start with the case \({\sigma }_1{\sigma }_2=1.\) By mass conservation \(|{\sigma }_1+{\sigma }_2|=|{\varvec{{\beta }}}_0-{\varvec{{\alpha }}}_0|=2.\) By (C.12) \(N\ge 4.\) Then conditions (7.1) and (C.12) imply that
Then
since \(N\ge 4\) and \(\widehat{n}_\ell \ge 1.\) When \({\sigma }_1{\sigma }_2=-1\) we have \(m_1\ne m_2\), \(|\pi |=|m_1-m_2|\ge 1\) and by mass conservation \({\varvec{{\alpha }}}_0-{\varvec{{\beta }}}_0=0.\) Then
If \(|m_1|>|m_2|\) then
Otherwise \(m_1=-m_2\) and, therefore, \(|\pi |=2|m_1|,\) completing the proof in the case \(D=2.\)
Let us now consider the case \(D \ge 3\). By (7.1), (C.11) and (C.12)
If \(\sigma _1\sigma _2 = 1\) then
If \({\sigma }_1{\sigma }_2 = -1\)
Now, if \({\left| m_1\right| }\ne {\left| m_2\right| }\) then
Conversely, if \({\left| m_1\right| } = {\left| m_2\right| }\), by (C.13), \(m_1\ne m_2\), hence \(m_1 = - m_2\). By substituting this relation into (C.10), we have
concluding the proof. \(\square \)
Conclusion of the proof of Lemma 7.1
As above, given \({\varvec{{\alpha }}},{\varvec{{\beta }}}\in {\mathbb {N}}^\mathbb {Z},\) with \(1\le |{\varvec{{\alpha }}}|=|{\varvec{{\beta }}}|<\infty \) we consider \(m=m({\varvec{{\alpha }}}-{\varvec{{\beta }}})\) and \(\widehat{n}=\widehat{n}({\varvec{{\alpha }}}+{\varvec{{\beta }}}).\) Note that \(N:=|{\varvec{{\alpha }}}+{\varvec{{\beta }}}|\ge 2.\)
We haveFootnote 33
using that \(1-\theta \le 2-2^\theta \) for \(0\le \theta \le 1.\) Then by Lemma 6.1 and (C.18) we get
proving (7.2).
Let us now prove (7.3) passing to the logarithm. We have
using that \(1+cx \le \frac{3}{2} x^c\) for \(c\ge 1,\)\(x\ge 2.\) If \({\varvec{{\alpha }}}_i-{\varvec{{\beta }}}_i=0\) for every \(|i|\ge 2\) then (7.3) follows. Assume now that \({\varvec{{\alpha }}}_i-{\varvec{{\beta }}}_i\ne 0\) for some \(|i|\ge 2.\) By (C.14) we have
We claim that, when \(N\ge 3,\)
Let \({\mathcal {S}}:=\{3\le l\le N,\ \mathrm{s.t.}\ \widehat{n}_l\ge 2\}.\) If \({\mathcal {S}}=\emptyset \) we have the equality in (C.21). Otherwise \(\sum _{l\in {\mathcal {S}}}\widehat{n}_l^2\ge 4\) andFootnote 34
proving (C.21).
We claim that
Indeed consider first the case \(\pi =0,\) then \(N\ge 3\) by (C.20) and (C.22) follows by (C.21). Consider now the case \(|\pi |\ge 1.\) If \(N<3\) (C.22) follows (there is no sum). If \(N\ge 3\) we haveFootnote 35
Recalling (C.21) this complete the proof of (C.22).
Let us continue the proof of (7.3). Set \(g(i):=0\) if \(|i|\le 1\) and \(g(i):=\ln |i|\) if \(|i|\ge 2\) and apply (C.9) to (C.19); we get
Inserting in (C.19) we obtain
concluding the proof of (7.3). \(\square \)
Proof of Lemma 7.2
First of all we note that
since \(f_i(0)=0.\) We have thatFootnote 36
We have that
where
since the maximum is achieved for \(x=1\) if \(\langle i \rangle \ge i_0\) and \(x=\frac{2C_* }{{\sigma }\langle i \rangle ^{\theta /2}}\) if \(\langle i \rangle < i_0\). Note that \(i_0\ge e.\) Then we get
We immediately have that
Moreover, in the case \(\langle i \rangle \ge i_0\ge e,\)
where
Therefore
satisfies
We have thatFootnote 37
Note that
Therefore
where
and \(M_\ell :=0\) if \(|\ell _i|=0\) for every \(|i|\ge i_\sharp .\) In conclusion we get
noting that \(\widehat{n}_1(\ell )=M_\ell \) if \(M_\ell \ne 0,\) otherwise \(\widehat{n}_1(\ell )< i_\sharp ,\) and, therefore,
\(\square \)
Proof of Lemma 4.1
For \(\ell \in \mathbb {Z}^\mathbb {Z}\) with \( 0<|\ell |<\infty \) we define
if \(\ell \) is such that \(\ell _n=0\)\(\forall n\ne 0\) then
$$\begin{aligned} \mu ({\mathcal {R}}_\ell ) = \frac{\gamma }{1+|\ell _0|^{\mu _1} }. \end{aligned}$$Otherwise: let \(s=s(\ell )>0\) be the smallest positive index i such that \(|\ell _i |+|\ell _{-i}|\ne 0\) and \(S=S(\ell )\) be the biggest. Then we haveFootnote 38
$$\begin{aligned} \mu ({\mathcal {R}}_\ell ) \le \frac{\gamma s^{q}}{{\left( 1+|\ell _0|^{\mu _1}\right) } }\prod _{n\ne 0}\frac{1}{(1+|\ell _n|^{\mu _1} |n|^{{\mu _2}+{q}})}. \end{aligned}$$
Let us write
Now
Let us estimate (C.24)
Now since
we have
Then we have
and consequently (C.24) is bounded by
Regarding the third line in (C.23), we note that for all n we have
Hence
Then, multiplying by \(\gamma s^{q}\) and taking the \(\sum _{0<s<S},\) we have that also (C.25) is bounded by some constant \({C_{\mathtt {meas}}}(\mu _1,\mu _2)\gamma \). \(\square \)
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Biasco, L., Massetti, J.E. & Procesi, M. An Abstract Birkhoff Normal Form Theorem and Exponential Type Stability of the 1d NLS. Commun. Math. Phys. 375, 2089–2153 (2020). https://doi.org/10.1007/s00220-019-03618-x
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DOI: https://doi.org/10.1007/s00220-019-03618-x