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A classification of martingale Hardy spaces associated withrearrangement-invariant function spaces
Authors:M.?Kikuchi  author-information"  >  author-information__contact u-icon-before"  >  mailto:kikuchi@sci.toyama-u.ac.jp"   title="  kikuchi@sci.toyama-u.ac.jp"   itemprop="  email"   data-track="  click"   data-track-action="  Email author"   data-track-label="  "  >Email author
Affiliation:(1) Department of Mathematics, Toyama University, Gofuku 3190, 930-8555 Toyama, Japan
Abstract:Let X be a rearrangement-invariant Banach function space over a complete probability space 
$$ (Omega,,Sigma,,mathbb{P}) $$
, and denote by 
$$ mathbb{H}(X,,({cal F}_n)) $$
the Hardy space consisting of all martingales 
$$ f = (f_{n},, {cal F}_{n})_{n geq 0} $$
such that 
$$ suplimits_{n geq 0} |f_n| in X $$
. We prove that 
$$  f = (f_{n},, {cal F}_n) in mathbb{H}(X,,({cal F}_n)) $$
implies
$$  {cal A}f = ({cal A}f_{n},, {cal F}_n) in mathbb{H}(X,,({cal F}_n)) $$
for any filtration 
$$ {cal F} = ({cal F}_n) $$
if and only if Doobrsquos inequality holds in X, where 
$$ {cal A}f = ({cal A}f_{n},, {cal F}_n) $$
denotes the martingale defined by 
$$ {cal A}f_{n} = mathbb{E}[|f_{infty}| , |{cal F}_n] $$
, n = 0, 1, 2, ..., and 
$$ f_{infty} = limlimits_{n to infty} f_n $$
a.s.Received: 1 August 2000
Keywords:60G42  60G46  46E30
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