Vakhitov–Kolokolov stability criterion - Wikiwand

# Vakhitov–Kolokolov stability criterion

The Vakhitov–Kolokolov stability criterion is a condition for linear stability (sometimes called spectral stability) of solitary wave solutions to a wide class of U(1)-invariant Hamiltonian systems, named after Soviet scientists Aleksandr Kolokolov (Александр Александрович Колоколов) and Nazib Vakhitov (Назиб Галиевич Вахитов). The condition for linear stability of a solitary wave ${\displaystyle u(x,t)=\phi _{\omega }(x)e^{-i\omega t))$ with frequency ${\displaystyle \omega }$ has the form

${\displaystyle {\frac {d}{d\omega ))Q(\omega )<0,}$

where ${\displaystyle Q(\omega )\,}$ is the charge (or momentum) of the solitary wave ${\displaystyle \phi _{\omega }(x)e^{-i\omega t))$, conserved by Noether's theorem due to U(1)-invariance of the system.

## Original formulation

Originally, this criterion was obtained for the nonlinear Schrödinger equation,

${\displaystyle i{\frac {\partial }{\partial t))u(x,t)=-{\frac {\partial ^{2)){\partial x^{2))}u(x,t)+g(|u(x,t)|^{2})u(x,t),}$

where ${\displaystyle x\in \mathbb {R} }$, ${\displaystyle t\in \mathbb {R} }$, and ${\displaystyle g\in C^{\infty }(\mathbb {R} )}$ is a smooth real-valued function. The solution ${\displaystyle u(x,t)\,}$ is assumed to be complex-valued. Since the equation is U(1)-invariant, by Noether's theorem, it has an integral of motion, ${\displaystyle Q(u)={\frac {1}{2))\int _{\mathbb {R} }|u(x,t)|^{2}\,dx}$, which is called charge or momentum, depending on the model under consideration. For a wide class of functions ${\displaystyle g}$, the nonlinear Schrödinger equation admits solitary wave solutions of the form ${\displaystyle u(x,t)=\phi _{\omega }(x)e^{-i\omega t))$, where ${\displaystyle \omega \in \mathbb {R} }$ and ${\displaystyle \phi _{\omega }(x)}$ decays for large ${\displaystyle x}$ (one often requires that ${\displaystyle \phi _{\omega }(x)}$ belongs to the Sobolev space ${\displaystyle H^{1}(\mathbb {R} ^{n})}$). Usually such solutions exist for ${\displaystyle \omega }$ from an interval or collection of intervals of a real line. The Vakhitov–Kolokolov stability criterion,[1][2][3][4]

${\displaystyle {\frac {d}{d\omega ))Q(\phi _{\omega })<0,}$

is a condition of spectral stability of a solitary wave solution. Namely, if this condition is satisfied at a particular value of ${\displaystyle \omega }$, then the linearization at the solitary wave with this ${\displaystyle \omega }$ has no spectrum in the right half-plane.

This result is based on an earlier work[5] by Vladimir Zakharov.

## Generalizations

This result has been generalized to abstract Hamiltonian systems with U(1)-invariance.[6] It was shown that under rather general conditions the Vakhitov–Kolokolov stability criterion guarantees not only spectral stability but also orbital stability of solitary waves.

The stability condition has been generalized[7] to traveling wave solutions to the generalized Korteweg–de Vries equation of the form

${\displaystyle \partial _{t}u+\partial _{x}^{3}u+\partial _{x}f(u)=0\,}$.

The stability condition has also been generalized to Hamiltonian systems with a more general symmetry group.[8]

## References

1. ^ Колоколов, А. А. (1973). "Устойчивость основной моды нелинейного волнового уравнения в кубичной среде". Прикладная механика и техническая физика (3): 152–155.
2. ^ A.A. Kolokolov (1973). "Stability of the dominant mode of the nonlinear wave equation in a cubic medium". Journal of Applied Mechanics and Technical Physics. 14 (3): 426–428. Bibcode:1973JAMTP..14..426K. doi:10.1007/BF00850963.
3. ^ Вахитов, Н. Г. & Колоколов, А. А. (1973). "Стационарные решения волнового уравнения в среде с насыщением нелинейности". Известия высших учебных заведений. Радиофизика. 16: 1020–1028.
4. ^ N.G. Vakhitov & A.A. Kolokolov (1973). "Stationary solutions of the wave equation in the medium with nonlinearity saturation". Radiophys. Quantum Electron. 16 (7): 783–789. Bibcode:1973R&QE...16..783V. doi:10.1007/BF01031343.
5. ^ Vladimir E. Zakharov (1967). "Instability of Self-focusing of Light" (PDF). Zh. Eksp. Teor. Fiz. 53: 1735–1743. Bibcode:1968JETP...26..994Z.
6. ^ Manoussos Grillakis; Jalal Shatah & Walter Strauss (1987). "Stability theory of solitary waves in the presence of symmetry. I". J. Funct. Anal. 74: 160–197. doi:10.1016/0022-1236(87)90044-9.
7. ^ Jerry Bona; Panagiotis Souganidis & Walter Strauss (1987). "Stability and instability of solitary waves of Korteweg-de Vries type". Proceedings of the Royal Society A. 411 (1841): 395–412. Bibcode:1987RSPSA.411..395B. doi:10.1098/rspa.1987.0073.
8. ^ Manoussos Grillakis; Jalal Shatah & Walter Strauss (1990). "Stability theory of solitary waves in the presence of symmetry". J. Funct. Anal. 94 (2): 308–348. doi:10.1016/0022-1236(90)90016-E.