This paper deals with spectral assertions of Riesz potentials in some classes of quasimetric spaces. In addition we survey briefly a few related subjects: integral operators, local means and function spaces, euclidean charts of quasi–metric spaces, relations to fractal geometry.

Let μ be a Radon measure on Rd which satisfies the growth condition only namely, there is a constant C > 0 such that for all x ∈ Rd,r > 0 and for some fixed 0 < n ≤d, μ(B(x, r)) ≤Crn, where B(x, r) is the ball centered at x and having radius r. In this paper, we first give a new atomic characterization of the Hardy space H1(μ) introduced by X. Tolsa. As applications of this new characterization, we establish the (H1(μ), L1,∞(μ)) estimate of the commutators generated by RBMO(μ) functions with the Calderon–Zygmund operators whose kernels satisfy only the size condition and a certain minimum regularity condition. Using this endpoint estimate and a new interpolation theorem for operators which is also established in this paper and has independent interest, we further obtain the Lp(μ) (1 < p < ∞) boundedness of these commutators.

Let (X,ρ,μ)dθbe a space of homogeneous type with d > 0 and θ∈(0,1],b be a paraaccretive function, ∈∈(0, θ]，︱s︱ < ∈, and a0 ∈(0, 1 be some constant depending on d, ∈ and s. The authors introduce the Besov space bBspq (X) with a0 < p ≤∞, and the Triebel-Lizorkin space bFspq(X) with a0 < p < ∞ and a0 < q≤∞ by first establishing a Plancherel-Polyatype inequality. Moreover, the authors establish the frame and the Littlewood-Paley function characterizations of these spaces. Furthermore, the authors introduce the new Besov space b−1 BPspq (X) and the Triebel-Lizorkin space b−1 FPspq (X). The relations among these spaces and the known Hardy-type spaces are presented. As applications, the authors also establish some real interpolation theorems, embedding theorems, Tb theorems, and the lifting property by introducing some new Riesz operators of these spaces.

New spaces of Lipschitz type on metric-measure spaces are introduced and they are shown to be just the well-known Besov spaces or Triebel–Lizorkin spaces when the smooth index is less than 1. These theorems also hold in the setting of spaces of homogeneous type, which include Euclidean spaces, Riemannian manifolds and some self-similar fractals. Moreover, the relationships amongst these Lipschitz-type spaces, Hajlasz–Sobolev spaces, Korevaar–Schoen–Sobolev spaces, Newtonian Sobolev space and Cheeger–Sobolev spaces on metric-measure spaces are clarified, showing that they are the same space with equivalence of norms. Furthermore, a Sobolev embedding theorem, namely that the Lipschitz-type spaces with large orders of smoothness can be embedded in Lipschitz spaces, is proved. For metric-measure spaces with heat kernels, a Hardy–Littlewood–Sobolev theorem is establish, and hence it is proved that Lipschitz-type spaces with small orders of smoothness can be embedded in Lebesgue spaces.

Suppose that μ is a Radon measure on Rd, which may be nondoubling. The only condition on μ is the growth condition, namely, there is a constant C0 > 0 such that for all x ∈ supp (μ) and r > 0,μ(B(x, r)) ≤ C0rn,where 0 < n ≤ d. In this paper, the authors establish a theory of Besov spaces Bspq(μ) for 1 ≤ p, q ≤ ∞ and |s| < θ, where θ > 0 is a real number which depends on the non-doubling measure μ, C0, n and d. The method used to define these spaces is new even for the classical case. As applications, the lifting properties of these spaces by using the Riesz potential operators and the dual spaces are obtained.