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arXiv 2004.05791nlin.SIhep-thmath-phmath.MP
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拉克斯对与零曲率表示

An introduction to Lax pairs and the zero curvature representation

Govind S. Krishnaswami, T. R. Vishnu

摘要:

拉克斯对是寻找某些动力系统守恒量的有用工具。在这篇简要的文章中,我们给出了一个有动机的介绍,一个Lax对矩阵$(L,A)$的想法,首先为机械系统,如线性谐振子,户田链,欧拉刚体和Rajeev-Ranken模型。然后,这是扩展到Lax运营商的一维场论,如线性波和KdV方程和重新制定为零曲率表示通过一个$(U,V)$对,这是说明使用非线性薛定谔方程。其关键思想是实现一个(可能)非线性演化方程作为一对线性方程之间的相容性条件。后者可以是Lax算子$L$的本征值问题和由$A$生成的线性演化方程,对于相应的本征函数。或者,它们可以是一阶线性系统,说明任意矢量关于1+1维规范势$(V,U)$的协变常数。相容性条件是相应规范势的Lax方程$\dot L = [L, A]$或平坦性条件$U_t - V_x + [U, V] = 0$。守恒量然后从Lax和monodromy矩阵的等谱性得出。

英文摘要:

Lax pairs are a useful tool in finding conserved quantities of some dynamical systems. In this expository article, we give a motivated introduction to the idea of a Lax pair of matrices $(L,A)$, first for mechanical systems such as the linear harmonic oscillator, Toda chain, Eulerian rigid body and the Rajeev-Ranken model. This is then extended to Lax operators for one-dimensional field theories such as the linear wave and KdV equations and reformulated as a zero curvature representation via a $(U,V)$ pair which is illustrated using the nonlinear Schrödinger equation. The key idea is that of realizing a (possibly) nonlinear evolution equation as a compatibility condition between a pair of linear equations. The latter could be an eigenvalue problem for the Lax operator $L$ and a linear evolution equation generated by $A$, for the corresponding eigenfunction. Alternatively, they could be the first order linear system stating the covariant constancy of an arbitrary vector with respect to the 1+1 dimensional gauge potential $(V,U)$. The compatibility conditions are then either the Lax equation $\dot L = [L, A]$ or the flatness condition $U_t - V_x + [U, V] = 0$ for the corresponding gauge potential. The conserved quantities then follow from the isospectrality of the Lax and monodromy matrices.

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