Stationary Grading and Heat-Transported Moving Boundaries: From the Cubic Case to Two Fourth-Order Branches
We study zeros transported by the heat semigroup from stationary solutions of the sparse operators $L_n=c_{n,0}\D^n+c_{0,n-1}x^{n-1}$. The starting point is the exact cubic boundary obtained by Hernández-del-Valle and Guerra-Polania~\cite{HerGuerra}. We show that the hypergeometric function used there is the residue-$1$ component of a naturally graded stationary solution space, and that heat transport preserves this grading after the scaling $x=t^ny$. This identifies the cubic boundary as the transported zero of a specific stationary mode. We then study $L_4=c_{4,0}\D^4+c_{0,3}x^3$. Its $6\times6$ boundary-jet determinant leads to the universal scaling \[ F(t)=ρ\left(\frac t2\right)^4 Φ\!\left(ρ^2\left(\frac t2\right)^7\right), \qquad ρ=\frac{c_{0,3}}{c_{4,0}}. \] The resulting nonlinear equation for $Φ$ is singular at the origin. After an explicit desingularization, its initial compatibility condition factors as \[ (5Φ'(0)-1)(6Φ'(0)-1)=0. \] Consequently there are exactly two normalized graded analytic boundary germs. We place the desingularized equation in Briot--Bouquet form, prove local existence and uniqueness of both branches, and give a triangular recursion for all their Taylor coefficients. Finally, we prove that the two branches are generated respectively by the residue-$1$ and residue-$3$ stationary solutions. The quadratic and cubic cases provide the established baseline; the fourth-order problem is the first point at which distinct stationary modes produce distinct analytic branches. We do not claim a general branch-count theorem.