An integral of a function with values in a Banach space with respect to a scalar measure. It belongs to the so-called strong integrals (cf. Strong integral).
Let be the vector space of functions , , with values in a Banach space , given on a space with a countably-additive scalar measure on a -algebra of subsets of . A function is called simple if
A function is called strongly measurable if there exists a sequence of simple functions with almost-everywhere with respect to the measure on . In such a case the scalar function is -measurable. For the simple function
A function is said to be Bochner integrable if it is strongly measurable and if for some approximating sequence of simple functions
The Bochner integral of such a function over a set is
where is the characteristic function of , and the limit is understood in the sense of strong convergence in . This limit exists, and is independent of the choice of the approximation sequence of simple functions.
Criterion for Bochner integrability: For a strongly-measurable function to be Bochner integrable it is necessary and sufficient for the norm of this function to be integrable, i.e.
The set of Bochner-integrable functions forms a vector subspace of , and the Bochner integral is a linear operator on this subspace.
Properties of Bochner integrals:
2) A Bochner integral is a countably-additive -absolutely continuous set-function on the -algebra , i.e.
if , and if , uniformly for .
3) If almost-everywhere with respect to the measure on , if almost-everywhere with respect to on , and if , then
4) The space is complete with respect to the norm (cf. Convergence in norm)
5) If is a closed linear operator from a Banach space into a Banach space and if
If is bounded, the condition
|||S. Bochner, "Integration von Funktionen, deren Werte die Elemente eines Vektorraumes sind" Fund. Math. , 20 (1933) pp. 262–276|
|||T.H. Hildebrandt, "Integration in abstract spaces" Bull. Amer. Math. Soc. , 59 (1953) pp. 111–139|
|||K. Yosida, "Functional analysis" , Springer (1980) pp. Chapt. 8, §1|
|||E. Hille, R.S. Phillips, "Functional analysis and semi-groups" , Amer. Math. Soc. (1957)|
|||N. Dunford, J.T. Schwartz, "Linear operators. General theory" , 1 , Interscience (1958)|
|[a1]||J. Diestel, J.J. Uhl jr., "Vector measures" , Math. Surveys , 15 , Amer. Math. Soc. (1977)|
|[a2]||A.C. Zaanen, "Integration" , North-Holland (1967)|
|[a3]||N. Bourbaki, "Elements of mathematics. Integration" , Addison-Wesley (1975) pp. Chapt.6;7;8 (Translated from French)|
|[a4]||J. Mikusiński, "The Bochner integral" , Acad. Press (1978)|
Bochner integral. V.I. Sobolev (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Bochner_integral&oldid=11334