Spherical functions and spectrum of the Laplacian on semi-homogeneous trees

Abstract

On a semi-homogeneous tree, we study the $\ell^p$-spectrum of the Laplace operator $\mu_1$ (the isotropic nearest-neighbor transition operator); the known results in the much simpler setting of homogeneous trees are obtained as particular cases. The spectrum is given by the eigenvalues of spherical functions, i.e., eigenfunctions of $mu_1$ that are radial with respect to a reference vertex $v_0$ and normalized there. We show that spherical functions are boundary integrals of generalized Poisson kernels that, unlike the homogeneous setting, are not complex powers of the usual Poisson kernel. We compute these generalized Poisson kernels via Markov chains and their generating functions, whence we work out explicit expressions for spherical functions. On semi-homogeneous trees, spherical functions turn out to have an $\ell^p$ behavior that does not occur on homogeneous trees: one of them, for an appropriate choice of $v_0$, belongs to $\ell^p$ for some $p<2$. Up to normalization, the operator $\mu_1^2$ differs from the step-2 Laplacian $\mu_2$ only by a shift. On the other hand, the recurrence relation associated to the semi-homogeneous $\mu_2$ is that of a polygonal graph, akin to that of the Laplacian in a homogeneous tree. By this token, we compute the spectra of $\mu_1^2$ on a semi-homogeneous tree, hence, by extracting square roots, the $\ell^p$-spectrum of $\mu_1$ for $1\leqslant p <\infty$, and show that it is disconnected for $p$ in an interval containing 2 but it is connected at all other values of $p$.