Conjectural tropical analog of Descartes' rule of signs

Let P(x)=k=0dakxkP(x)=\sum_{k=0}^d a_kx^k be a real univariate polynomial, and define the tropical polynomial associated to PP by

trP(ξ)=max0kd(kξ+lnakk2),ξR,tr^\dagger_P(\xi)=\max_{0\le k\le d}(k\xi+\ln|a_k|-k^2),\qquad \xi\in\mathbb R,

where the weights are given by λk=ek2\lambda_k^\dagger=e^{-k^2} for k=0,1,k=0,1,\dots. A positive or negative tropical root is understood to be an essential tropical root with the corresponding sign.

Conjectural tropical analog of Descartes' rule of signs. For any real univariate polynomial P(x)P(x), the number of its positive (negative) roots does not exceed the number of positive (negative) essential tropical roots of trP(ξ)tr^\dagger_P(\xi).

This conjecture proposes a tropical analogue of Descartes' rule of signs for the fixed rapidly decreasing weight sequence λk=ek2\lambda_k^\dagger=e^{-k^2}. The preceding results establish sufficient and necessary-type log-concavity conditions for real-to-tropical root preservers, but do not settle this particular sequence.

Sources & referencesView supporting material

Primary source

J. Forsgård, D. Novikov and B. Shapiro, “A tropical analog of Descartes' rule of signs”, arXiv:1510.03257 (2015).

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