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 New York Journal of Mathematics 8 (2002), 9-30.
 Published: January 26, 2002 Keywords: Meromorphic solutions of differential equations, KdV-hierarchy, AKNS-hierarchy, Gelfand-Dikii-hierarchy Subject: 34M05, 37K10
 Abstract: If the differential expressions $P$ and $L$ are polynomials (over $\bb C$) of another differential expression they will obviously commute. To have a $P$ which does not arise in this way but satisfies $[P,L]=0$ is rare. Yet the question of when it happens has received a lot of attention since Lax presented his description of the KdV hierarchy by Lax pairs $(P,L)$. In this paper the question is answered in the case where the given expression $L$ has matrix-valued coefficients which are rational functions bounded at infinity or simply periodic functions bounded at the end of the period strip: if $Ly=zy$ has only meromorphic solutions then there exists a $P$ such that $[P,L]=0$ while $P$ and $L$ are not both polynomials of any other differential expression. The result is applied to the AKNS hierarchy where $L=JD+Q$ is a first order expression whose coefficients $J$ and $Q$ are $2\times 2$ matrices. It is therefore an elementary exercise to determine whether a given matrix $Q$ with rational or simply periodic coefficients is a stationary solution of an equation in the AKNS hierarchy. Acknowledgments: Research supported in part by the US National Science Foundation under Grant No. DMS-9970299. Author information: Department of Mathematics, University of Alabama at Birmingham, Birmingham, Alabama 35294-1170, USA rudi@math.uab.edu http://www.math.uab.edu/rudi/