The 3-state Potts model on planar triangulations: explicit algebraic solution
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| Format: | Preprint |
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2025
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| author | Bousquet-Mélou, Mireille Notarantonio, Hadrien |
| author_facet | Bousquet-Mélou, Mireille Notarantonio, Hadrien |
| contents | We consider the $3$-state Potts generating function $T(ν,w)$ of planar triangulations; that is, the bivariate series that counts planar triangulations with vertices coloured in $3$ colours, weighted by their size (number of vertices, recorded by the variable $w$) and by the number of monochromatic edges (variable $ν$).
This series was proved to be algebraic 15 years ago by Bernardi and the first author: this follows from its link with the solution of a discrete differential equation (DDE), and from general algebraicity results on such equations. However, despite recent progresses on the effective solution of DDEs, the exact value of $T(ν,w)$ has remained unknown so far -- except in the case $ν=0$, corresponding to proper colourings and solved by Tutte in the sixties. We determine here this exact value, proving that $T(ν,w)$ satisfies a polynomial equation of degree $11$ in $T$ and genus $1$ in $w$ and $T$. We prove that the critical value of $ν$ is $ν_c=1+3/\sqrt{47}$, with a critical exponent $6/5$ in the series $T(ν_c, \cdot)$, while the other values of $ν$ yield the usual map exponent $3/2$.
By duality of the planar Potts model, our results also characterize the 3-state Potts generating function of planar cubic maps, in which all vertices have degree $3$. In particular, the annihilating polynomial, still of degree $11$, that we obtain for properly 3-coloured cubic maps proves a conjecture by Bruno Salvy from 2009. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2510_08414 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | The 3-state Potts model on planar triangulations: explicit algebraic solution Bousquet-Mélou, Mireille Notarantonio, Hadrien Combinatorics 05A15, 05C30, 05C31 We consider the $3$-state Potts generating function $T(ν,w)$ of planar triangulations; that is, the bivariate series that counts planar triangulations with vertices coloured in $3$ colours, weighted by their size (number of vertices, recorded by the variable $w$) and by the number of monochromatic edges (variable $ν$). This series was proved to be algebraic 15 years ago by Bernardi and the first author: this follows from its link with the solution of a discrete differential equation (DDE), and from general algebraicity results on such equations. However, despite recent progresses on the effective solution of DDEs, the exact value of $T(ν,w)$ has remained unknown so far -- except in the case $ν=0$, corresponding to proper colourings and solved by Tutte in the sixties. We determine here this exact value, proving that $T(ν,w)$ satisfies a polynomial equation of degree $11$ in $T$ and genus $1$ in $w$ and $T$. We prove that the critical value of $ν$ is $ν_c=1+3/\sqrt{47}$, with a critical exponent $6/5$ in the series $T(ν_c, \cdot)$, while the other values of $ν$ yield the usual map exponent $3/2$. By duality of the planar Potts model, our results also characterize the 3-state Potts generating function of planar cubic maps, in which all vertices have degree $3$. In particular, the annihilating polynomial, still of degree $11$, that we obtain for properly 3-coloured cubic maps proves a conjecture by Bruno Salvy from 2009. |
| title | The 3-state Potts model on planar triangulations: explicit algebraic solution |
| topic | Combinatorics 05A15, 05C30, 05C31 |
| url | https://arxiv.org/abs/2510.08414 |