Casimir-family criterion for generic Kronecker bihamiltonian structures

Let MM be a manifold with compatible Poisson structures {,}1\{,\}_{1} and {,}2\{,\}_{2}. Let LL be a finite set with rr elements, and let Fl,λF_{l,\lambda}, for lLl\in L and λC\lambda\in\Bbb C, be smooth functions on MM that are Casimirs of λ{,}1+{,}2\lambda\{,\}_{1}+\{,\}_{2}. Suppose

Fl,λ(m)=k=0dlfl,k(m)λk.F_{l,\lambda}(m)=\sum_{k=0}^{d_l}f_{l,k}(m)\lambda^k.

For mMm\in M, let W1(m)TmMW_1(m)\subset T_m^*M be spanned by the differentials dfl,kmdf_{l,k}|_m. If, at m0Mm_0\in M, dimW1(m0)(dimM+r)/2\dim W_1(m_0)\geq(\dim M+r)/2, if some combination λ1{,}1+λ2{,}2\lambda_1\{,\}_1+\lambda_2\{,\}_2 has at most rr independent Casimir functions near m0m_0, and if lL(2dl+1)dimM\sum_{l\in L}(2d_l+1)\leq\dim M, then dimMr\dim M-r is even, dimW1(m0)=(dimM+r)/2\dim W_1(m_0)=(\dim M+r)/2, 2lLdl+r=dimM2\sum_{l\in L}d_l+r=\dim M, and the bihamiltonian structure is Kronecker of type (2d1+1,,2dr+1)\left(2d_1+1,\dots,2d_r+1\right) on an open subset UMU\subset M whose closure contains m0m_0. Casimir-family criterion conjecture. Under these hypotheses, all the stated conclusions hold. The criterion would characterize Kronecker structures through the mutual position of Casimir functions for linear combinations of the two Poisson brackets. The source gives no resolution.

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Primary source

Israel M. Gelfand and Ilya Zakharevich, “Webs, Lenard schemes, and the local geometry of bihamiltonian Toda and Lax structures”, arXiv:math/9903080 (2000).

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