Continuous flow
2020 Mathematics Subject Classification: Primary: 37A10 [MSN][ZBL]
A continuous flow in ergodic theory is a family 
(where 
ranges over the real axis 
) of automorphisms modulo 0 of a measure space 
such that: a) 
for any 
and all 
, except possibly for a set of 
belonging to an exceptional set of measure 0 (which may depend on 
and 
); in other words, 
; b) for each measurable set 
the measure of the symmetric difference 
depends continuously on 
. Let 
be the set of all automorphisms modulo 0 of the space 
with the usual identification: if 
and 
coincide almost-everywhere, then they determine the same element of 
. If 
is endowed with the weak topology (see ), then b) means that the mapping 
that takes 
to 
is continuous.
If 
is a Lebesgue space, then the concept of a continuous flow is practically the same as that of a measurable flow: The latter is always a continuous flow (see ), and for any continuous flow 
there is a measurable flow 
such that 
for all 
(see ; a related result is proved in , but see also the correction in ). The converse to any of these results depends on the character of the problem in question and the methods used.
In another sense the term "continuous flow" can be used to emphasize that the flow is considered in the context of topological dynamics. In this meaning a continuous flow is a collection of homeomorphisms 
of a topological space 
such that 
for all 
and 
; the mapping 
taking 
to 
is continuous.
To avoid confusion with 1) it is better to talk in this case of a topological flow and in the case of 1) of a metric continuity.
References
| [1] | P.R. Halmos, "Lectures on ergodic theory" , Math. Soc. Japan (1956) |
| [2] | E. Hopf, "Ergodentheorie" , Springer (1970) |
| [3] | A.M. Vershik, "Measurable realization of continuous automorphism groups of a unitary ring" Izv. Akad. Nauk. SSSR Ser. Mat. , 29 : 1 (1965) pp. 127–136 |
| [4] | G.W. Mackey, "Point realizations of transformation groups" Illinois J. Math. , 6 : 2 (1962) pp. 327–335 |
| [5] | A. Ramsay, "Virtual groups and group actions" Advances in Math. , 6 : 3 (1971) pp. 253–322 |
Continuous flow. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Continuous_flow&oldid=23596