Li Guang, LIU Da Chun YANG, Dong Yong YANG
Let (
Y,
d,dλ) be (R
n, |·|,
μ), where |·| is the Euclidean distance,
μ is a nonnegative Radon measure on R
n satisfying the polynomial growth condition, or the Gauss measure metric space (R
n, |·|,
dγ), or the space (
S, d, ρ), where
S ≡ R
n R
+ is the (
ax + b)-group,
d is the left-invariant Riemannian metric and
ρ is the right Haar measure on
S with exponential growth. In this paper, the authors introduce and establish some properties of the atomic Hardy-type spaces {
Xs(
Y)}
0<s≤∞ and the BMO-type spaces {BMO(
Y, s)}
0<s<∞. Let
H1(
Y) be the known atomic Hardy space and
L01(
Y) the subspace of
f ∈
L1(
Y) with integral 0. The authors prove that the dual space of
Xs(
Y) is BMO(
Y, s) when
s ∈ (0,∞),
Xs(
Y) =
H1(
Y) when
s ∈ (0, 1], and
X∞(
Y) =
L01(
Y) (or
L1(
Y)). As applications, the authors show that if
T is a linear operator bounded from
H1(
Y) to
L1(
Y) and from
L1(
Y) to
L1,∞(
Y), then for all
r ∈ (1,∞) and
s ∈ (
r,∞],
T is bounded from
Xr(
Y) to the Lorentz space
L1,s(
Y), which applies to the Calderòn-Zygmund operator on (R
n, |·|,
μ), the imaginary powers of the Ornstein-Uhlenbeck operator on (R
n, |·|,
dγ) and the spectral operator associated with the spectral multiplier on (
S, d, ρ). All these results generalize the corresponding results of Sweezy, Abu-Shammala and Torchinsky on Euclidean spaces.