Spectral Selection in Sphere-Constrained Flows Generated by Polynomials of the Dirichlet Laplacian
Let $\Omega\subset\mathbb{R}^d$ be a bounded smooth domain and let $-\Delta_D$ be the positive Dirichlet Laplacian on $L^2(\Omega)$. For a real polynomial $p$ with positive leading coefficient, we study the constrained linear equation \[ u_t=-\Pi_u\,p(-\Delta_D)u, \qquad \lVert u(0)\rVert_{L^2}=1. \] Its solution is the normalized semigroup orbit \[ u(t)=\frac{e^{-t p(-\Delta_D)}u_0} {\lVert e^{-t p(-\Delta_D)}u_0\rVert_{L^2}}. \] The active spectral support is preserved, and the trajectory converges to the normalized projection of $u_0$ onto the active eigenspaces for which $p(\lambda_j)$ is minimal. The next active polynomial spectral value gives the exponential rate. For every $\theta\geq 0$ and $\tau>0$, the same rate holds in the domain of $(-\Delta_D)^\theta$ for $t\geq\tau$, even when the initial datum has no fractional regularity. We also show that a finite set of Dirichlet levels can be prescribed as the global minimizing set of a polynomial, and that an isolated selected set is stable under sufficiently small polynomial perturbations. For $p(s)=s^m$, the lowest active Dirichlet level is selected. For $p(s)=(s-\rho)^2$, selection is by distance from $\rho$, and cross-level degeneracy occurs only at Dirichlet midpoints.