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On $\sigma$-algebras related to the measurability of compositions

I. V. Shragin


Abstract: Given a measurable space $(T,\mathscr T)$, a set $X$, and a map $\varphi\colon T\to X$, the $\sigma$-algebras
$$ \mathfrak N_\varphi=\{B\subset X:\varphi^{-1}(B)\in\mathscr T\},\qquad \mathfrak M_\varphi=\{D\subset T\times X:G_\varphi^{-1}(D)\in\mathscr T\}, $$
$\mathfrak N_\Phi=\bigcap_{\varphi\in\Phi}\mathfrak N_\varphi$, and $\mathfrak M_\Phi=\bigcap_{\varphi\in\Phi}\mathfrak M_\varphi$, where $G_\varphi(t)=(t,\varphi(t))$ and $\Phi\subset X^T$, are considered. These $\sigma$-algebras are used to characterize the $(\mathscr T,\mathscr B)$-measurability of the compositions $g\circ\varphi$ and $f\circ G_\varphi$, where $g\colon X\to Y$, $f\colon T\times X\to Y$, and $(Y,\mathscr B)$ is a measurable space. Their elements are described without using the operations $\varphi^{-1}$ and $G_\varphi^{-1}$.

UDC: 517.51

Received: 20.10.2003

DOI: 10.4213/mzm3368

 English version: Mathematical Notes, 2006, 80:6, 868–874

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