Conditions of positivity on a shadow Markoff Tree
Nathan Bonin
TL;DR
This work analyzes positivity conditions for solutions of the shadow Markoff equation $A^2+B^2+C^2=(3-\sigmaoldsymbol{ abla}oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{oldsymbol{}}}}}}}}}}}})}ABC$ with dual numbers $A=a+oldsymbol{ extepsilon}oldsymbol{ extalpha}$, $B=b+oldsymbol{ extepsilon}oldsymbol{ extbeta}$, $C=c+oldsymbol{ extepsilon}oldsymbol{ extgamma}$ and $oldsymbol{ extepsilon}^2=0$. The author formulates a conjecture that positivity along mutations is governed by the rational point $(oldsymbol{ extalpha}:oldsymbol{ extbeta}:oldsymbol{ extgamma})$ lying in a convex polygon $P$ in ${ m RP}^2$, whose vertices are $(0:0:1),(1:0:2),(1:1:1),(0:2:1)$. A key theoretical advance is the linear dependence of the nilpotent part $oldsymbol{ extalpha}'$ on $(oldsymbol{ extalpha},oldsymbol{ extbeta},oldsymbol{ extgamma})$, enabling a convex-geometry analysis that underpins a main theorem tying positivity to membership in $P$. The paper then constructs four shadow Markoff trees corresponding to the polygon’s vertices, linking shadow sequences to classical integer sequences (e.g., Fibonacci, Pell) and providing Sage-based numerical evidence that supports the conjectured polygon as the positivity domain. Overall, the work offers a convex-geometric framework for positivity in shadow Markoff dynamics and connects this framework to classic Markoff theory and combinatorial sequences.
Abstract
An analogue of the Markoff equation has recently been introduced by the author and Valentin Ovsienko. A conjecture about the necessary and sufficient conditions for positivity of solutions to this equation is formulated and discussed.
