Bochner integral
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:
1)
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
and
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
then
If is bounded, the condition
is automatically fulfilled, [3]–[5].
The Bochner integral was introduced by S. Bochner [1]. Equivalent definitions were given by T. Hildebrandt [2] and N. Dunford (the -integral).
References
[1] | S. Bochner, "Integration von Funktionen, deren Werte die Elemente eines Vektorraumes sind" Fund. Math. , 20 (1933) pp. 262–276 |
[2] | T.H. Hildebrandt, "Integration in abstract spaces" Bull. Amer. Math. Soc. , 59 (1953) pp. 111–139 |
[3] | K. Yosida, "Functional analysis" , Springer (1980) pp. Chapt. 8, §1 |
[4] | E. Hille, R.S. Phillips, "Functional analysis and semi-groups" , Amer. Math. Soc. (1957) |
[5] | N. Dunford, J.T. Schwartz, "Linear operators. General theory" , 1 , Interscience (1958) |
Comments
A simple function is also called a step function. A good recent textbook on integrals with values in a Banach space is [a1]; [a4] is specifically about the Bochner integral.
References
[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. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Bochner_integral&oldid=11334