The continuous-limit correspondence between differential and q-difference spectral types

Consider an m×mm \times m Fuchsian differential system

dYdx=j=0nAjxujY,u0:=0.\frac{dY}{dx}=\sum_{j=0}^n\frac{\mathcal{A}_j}{x-u_j}Y,\qquad u_0:=0.

Let Pj=m1j,,mjj\mathcal{P}_j=m^j_1,\ldots,m^j_{\ell_j} for j=0,,n,j=0,\ldots,n,\infty be the partition of mm corresponding to Aj\mathcal{A}_j, so its spectral type is P0;;Pn;P\mathcal{P}_0;\ldots;\mathcal{P}_n;\mathcal{P}_\infty. Consider an m×mm\times m linear qq-difference system

Y(qx)=A(x)Y(x),A(x)=Anxn++A0,Y(qx)=A(x)Y(x),\qquad A(x)=A_nx^n+\cdots+A_0,

with spectral type S0=P0S_0=\mathcal{P}_0, S=PS_\infty=\mathcal{P}_\infty, and

Sdiv=m11,,m11,,m1n,,mnn.S_{\mathrm{div}}=m^1_1,\ldots,m^1_{\ell_1},\ldots,m^n_1,\ldots,m^n_{\ell_n}.

After changing the dependent variable by Z(x)=f(x)Y(x)Z(x)=f(x)Y(x) for a suitable scalar function f(x)f(x), the system becomes

Z(x)Z(qx)(1q)x=1(1q)x{Imf(qx)f(x)A(x)}Z(x).\frac{Z(x)-Z(qx)}{(1-q)x}=\frac{1}{(1-q)x}\left\{I_m-\frac{f(qx)}{f(x)}A(x)\right\}Z(x).

Continuous-limit correspondence. If, as q1q\to1, the zeros αjk\alpha^k_j of detA(x)\det A(x) corresponding to mjkm^k_j satisfy

αjkuk(j=1,,k),\alpha^k_j\to u_k\qquad (j=1,\ldots,\ell_k),

then the linear qq-difference equation can tend in the continuous limit q1q\to1 to the given Fuchsian differential equation. This proposes a correspondence between the spectral types of the two systems under the continuous limit. The claim concerns the proposed degeneration from the specified qq-difference system to the Fuchsian differential system; the source provides no resolution or further evidence of its status.

Sources & referencesView supporting material

Primary source

Hiroshi Kawakami, “A q-analogue of the matrix sixth Painlevé system”, arXiv:2001.08114 (2020).

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