# Pettis integral

An integral of a vector-valued function with respect to a scalar measure, which is a so-called weak integral. It was introduced by B.J. Pettis [1].

Let $F(X,E,\mathfrak B,\mu)$ be the vector space of functions $x(t)$, $t\in E$, with values in the Banach space $X$ and given on a set $(E,\mathfrak B,\mu)$ with a countably-additive measure $\mu$ on the $\sigma$-algebra $\mathfrak B$ of subsets of $E$. The function $x(t)$ is called weakly measurable if for any $f\in X^*$ the scalar function $f[x(t)]$ is measurable. The function $x(t)$ is Pettis integrable over a measurable subset $M\subset E$ if for any $f\in X^*$ the function $f[x(t)]$ is integrable on $M$ and if there exists an element $x(M)\in X$ such that

$$f[x(M)]=\int\limits_Mf[x(t)]d\mu.$$

Then, by definition,

$$\int\limits_Mx(t)d\mu=x(M)$$

is called the Pettis integral. That integral was introduced for the case $E=(a,b)$ with the ordinary Lebesgue measure by I.M. Gel'fand [2].

#### References

[1] | B.J. Pettis, "On integration in vector spaces" Trans. Amer. Math. Soc. , 44 : 2 (1938) pp. 277–304 |

[2] | I.M. Gel'fand, "Sur un lemme de la théorie des espaces linéaires" Zap. Naukovodosl. Inst. Mat. Mekh. Kharkov. Mat. Tov. , 13 : 1 (1936) pp. 35–40 |

[3] | T. Hildebrandt, "Integration in abstract spaces" Bull. Amer. Math. Soc. , 59 (1953) pp. 111–139 |

[4] | E. Hille, R.S. Phillips, "Functional analysis and semi-groups" , Amer. Math. Soc. (1957) |

#### Comments

#### References

[a1] | J. Diestel, J.J. Uhl jr., "Vector measures" , Math. Surveys , 15 , Amer. Math. Soc. (1977) |

[a2] | M. Talagrand, "Pettis integral and measure theory" , Mem. Amer. Math. Soc. , 307 , Amer. Math. Soc. (1984) |

[a3] | K. Bichteler, "Integration theory (with special attention to vector measures)" , Lect. notes in math. , 315 , Springer (1973) |

**How to Cite This Entry:**

Pettis integral.

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Pettis_integral&oldid=33082