The positive-characteristic intersection-of-orbits conjecture for polynomials

Let KK be a field of characteristic pp, and let f,gK[x]f,g\in K[x] be polynomials of degrees larger than 11. Suppose that there exist α,βK\alpha,\beta\in K such that Of(α)Og(β)\mathcal{O}_f(\alpha)\cap\mathcal{O}_g(\beta) is infinite. A polynomial is additive if it satisfies h(x+y)=h(x)+h(y)h(x+y)=h(x)+h(y), and a polynomial is linearly conjugate to hh if it has the form λ1hλ\lambda^{-1}\circ h\circ\lambda for a linear polynomial λ\lambda.

Intersection-of-orbits conjecture. At least one of the following holds: (1) there exist m,nNm,n\in{\mathbb N} such that fm=gnf^m=g^n; or (2) there exist linear polynomials λ,μK[x]\lambda,\mu\in\overline{K}[x] and additive polynomials f~,g~K[x]\widetilde f,\widetilde g\in\overline{K}[x] such that

f=λ1f~λ,g=μ1g~μ,f=\lambda^{-1}\circ\widetilde f\circ\lambda,\qquad g=\mu^{-1}\circ\widetilde g\circ\mu,

and there exists mNm\in{\mathbb N} such that

f~mg~m=g~mf~m.\widetilde f^m\circ\widetilde g^m=\widetilde g^m\circ\widetilde f^m.

This conjecture modifies the common-iterate conclusion known over characteristic zero to account for positive-characteristic counterexamples involving additive polynomials. Its full generality remains open, and no counterexamples are known.

Sources & referencesView supporting material

Primary source

Simone Coccia, Dragos Ghioca, Jungin Lee and Gyeonghyeon Nam, “Intersection of orbits for polynomials in characteristic p”, arXiv:2408.06937 (2024).

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