Remarks on the Yablonskii–Vorob'ev polynomials

It is well known that rational solutions of the second Painlevé equation (PII) are expressed in terms of logarithmic derivatives of the Yablonskii–Vorob'ev polynomials Qn(z) which are defined through a second order, bilinear differential-difference equation which is equivalent to the Toda equation. In this Letter, using the Hamiltonian theory for PII, it is shown that Qn(z) also satisfies a fourth order, bilinear ordinary differential equation and a fifth order, quad-linear difference equation. Further, rational solutions of some ordinary differential equations which are solvable in terms of solutions of PII are also expressed in terms of the Yablonskii–Vorob'ev polynomials.