Strong asymptotics conjecture for Padé polynomials under Stahl's theorem

Let fH()f\in\mathscr H(\infty) and fA0(CΣ)f\in\mathscr A^0(\mathbb C\setminus\Sigma) for some finite set ΣC\Sigma\subset\mathbb C. Let D=D(f)D=D(f) be Stahl's maximal domain associated with ff, S=S(f)=DS=S(f)=\partial D its Stahl compact set, and let R2=R2(f)\mathfrak R_2=\mathfrak R_2(f) be the canonical hyperelliptic two-sheeted Riemann surface associated with SS. Write z=z(1,2)=(z,±)R2\mathbf z=z^{(1,2)}=(z,\pm)\in\mathfrak R_2 for a point on this surface, and let Ψn(z)=Ψn(z;f)\Psi_n(\mathbf z)=\Psi_n(\mathbf z;f) be Nuttall's psi-function associated with ff and R2\mathfrak R_2. After a suitable normalization of the Padé polynomials Pn,j(z)=Pn,j(z;f)P_{n,j}(z)=P_{n,j}(z;f), j=0,1j=0,1, and the remainder function RnR_n, the following relations hold in capacity inside DD:

Strong asymptotics conjecture.

Pn,j(z)=cap(1)jfj(z)Ψn(z(1))(1+o(1)),n,P_{n,j}(z)\overset{\operatorname{cap}}=\frac{(-1)^j}{f^j(z)}\Psi_n(z^{(1)})\bigl(1+o(1)\bigr),\qquad n\to\infty, Rn(z)=capΨn(z(2))w(z(2))(1+o(1)),n,R_n(z)\overset{\operatorname{cap}}=\frac{\Psi_n(z^{(2)})}{w(z^{(2)})}\bigl(1+o(1)\bigr),\qquad n\to\infty,

where w2=H2g+2(z)w^2=H_{2g+2}(z), with H2g+2C2g+2[z]H_{2g+2}\in\mathbb C_{2g+2}[z], is the equation determining the hyperelliptic Riemann surface R2\mathfrak R_2 of genus gg.

These relations describe the expected strong asymptotics of Padé polynomials and their remainder under the conditions of Stahl's theorem. In general, except in genus zero, such a representation need not be unique because of spurious zeros. The source presents this as a conjectural form rather than an established theorem.

Sources & referencesView supporting material

Primary source

Nikolay R. Ikonomov, Ralitza K. Kovacheva and Sergey P. Suetin, “On the limit zero distribution of type I Hermite-Padé polynomials”, arXiv:1506.08031 (2015).

Additional references

2 papers in this index state this conjecture (2011–2015). The statement above is taken from the most recent of them; the others are arXiv:1101.2589.

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