Experimental mathematics on the magnetic susceptibility of the square lattice Ising model.

*(English)* Zbl 1152.82305
Summary: We calculate very long low- and high-temperature series for the susceptibility $\chi$ of the square lattice Ising model as well as very long series for the five-particle contribution $\chi^{(5)}$ and six-particle contribution $\chi^{(6)}$. These calculations have been made possible by the use of highly optimized polynomial time modular algorithms and a total of more than 150 000 CPU hours on computer clusters. The series for $\chi$ (low- and high-temperature regimes), $\chi^{(5)}$ and $\chi^{(6)}$ are now extended to 2000 terms. In addition, for $\chi^{(5)}$, 10 000 terms of the series are calculated modulo a single prime, and have been used to find the linear ODE satisfied by $\chi^{(5)}$ modulo a prime. A diff-Padé analysis of the 2000 terms series for $\chi^{(5)}$ and $\chi^{(6)}$ confirms to a very high degree of confidence previous conjectures about the location and strength of the singularities of the n-particle components of the susceptibility, up to a small set of `additional’ singularities. The exponents at all the singularities of the Fuchsian linear ODE of $\chi^{(5)}$ and the (as yet unknown) ODE of $\chi^{(6)}$ are given: they are all rational numbers. We find the presence of singularities at $w=1/2$ for the linear ODE of $\chi^{(5)}$, and $w^2=1/8$ for the ODE of $\chi^{(6)}$, which are not singularities of the `physical’ $\chi^{(5)}$ and $\chi^{(6)}$, that is to say the series solutions of the ODE’s which are analytic at $w=0$. Furthermore, analysis of the long series for $\chi^{(5)}$ (and $\chi^{(6)}$) combined with the corresponding long series for the full susceptibility $\chi$ yields previously conjectured singularities in some $\chi^{(n)}$, $n\ge7$. The exponents at all these singularities are also seen to be rational numbers. We also present a mechanism of resummation of the logarithmic singularities of the $\chi^{(n)}$ leading to the known power-law critical behaviour occurring in the full $\chi$, and perform a power spectrum analysis giving strong arguments in favour of the existence of a natural boundary for the full susceptibility $\chi$.

##### MSC:

82B20 | Lattice systems (Ising, dimer, Potts, etc.) and systems on graphs |

34M55 | Painlevé and other special equations; classification, hierarchies |

47E05 | Ordinary differential operators |

81Qxx | General mathematical topics and methods in quantum theory |

32G34 | Moduli and deformations for ordinary differential equations |

82-05 | Experimental papers (statistical mechanics) |