Finite-Branch Multipliers on a Hölder Algebra: Commutants, Norm Closures, and Essential Spectra

Abstract

Noninjective multiplication symbols allow commuting operators to mix function values along their fibers, while Hölder regularity constrains this mixing at branch points. We study these operators on the Banach algebra $A_{\beta }$, $0<\beta <1$, defined by a restricted interval-variation condition. For $u(t)=t(1-t)$, we determine the full commutant $\{M_u{\}}^{\prime }={\mathcal{C}}_{\beta }$, including operators whose reflection coefficients need not belong to $A_{\beta }$. Separately, we characterize the operator-norm closure $J_{\beta }$ of the regular multiplication--reflection subalgebra of ${\mathcal{C}}_{\beta }$, obtain two-sided norm-distance estimates, and find a complemented copy of ${\ell }^{\infty }/c_0$ in the Banach-space quotient ${\mathcal{C}}_{\beta }/J_{\beta }$. Every $T\in {\mathcal{C}}_{\beta }$ satisfies $\sigma (T)={\sigma }_{\mathrm{ap}}(T)={\sigma }_{\mathrm{usf}}(T)={\sigma }_{\mathrm{ess}}(T)$, and Fredholmness is equivalent to invertibility. For multipliers with finitely many full bi-Lipschitz branches, we also characterize their full commutants and establish the same spectral equalities for every commuting operator. For the regular finite-branch algebra, we establish an exact norm-closure criterion using weighted local Hölder defects and separate row-difference conditions, even when several folds share a level.