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On invariant measures for power bounded positive operators

Ryotaro Sato — 1996

Studia Mathematica

We give a counterexample showing that ( I - T * ) L ¯ L + = 0 does not imply the existence of a strictly positive function u in L 1 with Tu = u, where T is a power bounded positive linear operator on L 1 of a σ-finite measure space. This settles a conjecture by Brunel, Horowitz, and Lin.

Pointwise ergodic theorems in Lorentz spaces L(p,q) for null preserving transformations

Ryotaro Sato — 1995

Studia Mathematica

Let (X,ℱ,µ) be a finite measure space and τ a null preserving transformation on (X,ℱ,µ). Functions in Lorentz spaces L(p,q) associated with the measure μ are considered for pointwise ergodic theorems. Necessary and sufficient conditions are given in order that for any f in L(p,q) the ergodic average n - 1 i = 0 n - 1 f τ i ( x ) converges almost everywhere to a function f* in L ( p 1 , q 1 ] , where (pq) and ( p 1 , q 1 ] are assumed to be in the set ( r , s ) : r = s = 1 , o r 1 < r < a n d 1 s , o r r = s = . Results due to C. Ryll-Nardzewski, S. Gładysz, and I. Assani and J. Woś are generalized and unified...

Vector-valued ergodic theorems for multiparameter additive processes

Ryotaro Sato — 1999

Colloquium Mathematicae

Let X be a reflexive Banach space and (Ω,Σ,μ) be a σ-finite measure space. Let d ≥ 1 be an integer and T=T(u):u=( u 1 , ... , u d ) , u i ≥ 0, 1 ≤ i ≤ d be a strongly measurable d-parameter semigroup of linear contractions on L 1 ((Ω,Σ,μ);X). We assume that to each T(u) there corresponds a positive linear contraction P(u) defined on L 1 ((Ω,Σ,μ);ℝ) with the property that ∥ T(u)f(ω)∥ ≤ P(u)∥f(·)∥(ω) almost everywhere on Ω for all f ∈ L 1 ((Ω,Σ,μ);X). We then prove stochastic and pointwise ergodic theorems for a d-parameter...

A general differentiation theorem for superadditive processes

Ryotaro Sato — 2000

Colloquium Mathematicae

Let L be a Banach lattice of real-valued measurable functions on a σ-finite measure space and T= T t : t < 0 be a strongly continuous semigroup of positive linear operators on the Banach lattice L. Under some suitable norm conditions on L we prove a general differentiation theorem for superadditive processes in L with respect to the semigroup T.

Pointwise ergodic theorems for functions in Lorentz spaces L p q with p ≠ ∞

Ryotaro Sato — 1994

Studia Mathematica

Let τ be a null preserving point transformation on a finite measure space. Assuming τ is invertible, P. Ortega Salvador has recently obtained sufficient conditions for the almost everywhere convergence of the ergodic averages in L p q with 1 < p < ∞, 1 < q < ∞. In this paper we obtain necessary and sufficient conditions for the almost everywhere convergence, without assuming that τ is invertible and only assuming that p ≠ ∞.

On a vector-valued local ergodic theorem in L

Ryotaro Sato — 1999

Studia Mathematica

Let T = T ( u ) : u d + be a strongly continuous d-dimensional semigroup of linear contractions on L 1 ( ( Ω , Σ , μ ) ; X ) , where (Ω,Σ,μ) is a σ-finite measure space and X is a reflexive Banach space. Since L 1 ( ( Ω , Σ , μ ) ; X ) * = L ( ( Ω , Σ , μ ) ; X * ) , the adjoint semigroup T * = T * ( u ) : u d + becomes a weak*-continuous semigroup of linear contractions acting on L ( ( Ω , Σ , μ ) ; X * ) . In this paper the local ergodic theorem is studied for the adjoint semigroup T*. Assuming that each T(u), u d + , has a contraction majorant P(u) defined on L 1 ( ( Ω , Σ , μ ) ; ) , that is, P(u) is a positive linear contraction on L 1 ( ( Ω , Σ , μ ) ; ) such that T ( u ) f ( ω ) P ( u ) f ( · ) ( ω ) almost everywhere...

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