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Packo
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Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).

Edit: However, I'm willing to impose weaker conditions on $(Y,\Sigma_Y)$ than being countably separatednessseparated, e.g. $(Y,\Sigma_Y)$ being allowed to be only separated or $\Sigma_Y$ containing all singletons $\{y\}$ for $y \in Y$.

The reason is the following simple counter example: $X:=Y:=\{1,2\}$ with $$\Sigma_X:=\{\emptyset,X, \{1\}, \{2\}\}, \qquad \Sigma_Y:=\{\emptyset,Y\} $$ and $f:=\mathrm{id}$. Then we have: $$ \Sigma_X\otimes\Sigma_Y = \{ \emptyset, X \times Y, \{1\} \times Y, \{2\} \times Y \}, $$ but $$\Gamma(f) =\{ (1,1), (2,2) \} \notin \Sigma_X\otimes\Sigma_Y. $$

Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).

However, I'm willing to impose weaker conditions on $(Y,\Sigma_Y)$ than being countably separatedness, e.g. $(Y,\Sigma_Y)$ being allowed to be only separated or $\Sigma_Y$ containing all singletons $\{y\}$ for $y \in Y$.

Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).

Edit: However, I'm willing to impose weaker conditions on $(Y,\Sigma_Y)$ than being countably separated, e.g. $(Y,\Sigma_Y)$ being allowed to be only separated or $\Sigma_Y$ containing all singletons $\{y\}$ for $y \in Y$.

The reason is the following simple counter example: $X:=Y:=\{1,2\}$ with $$\Sigma_X:=\{\emptyset,X, \{1\}, \{2\}\}, \qquad \Sigma_Y:=\{\emptyset,Y\} $$ and $f:=\mathrm{id}$. Then we have: $$ \Sigma_X\otimes\Sigma_Y = \{ \emptyset, X \times Y, \{1\} \times Y, \{2\} \times Y \}, $$ but $$\Gamma(f) =\{ (1,1), (2,2) \} \notin \Sigma_X\otimes\Sigma_Y. $$

added 230 characters in body
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Packo
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Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).

However, I'm willing to impose weaker conditions on $(Y,\Sigma_Y)$ than being countably separatedness, e.g. $(Y,\Sigma_Y)$ being allowed to be only separated or $\Sigma_Y$ containing all singletons $\{y\}$ for $y \in Y$.

Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).

Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).

However, I'm willing to impose weaker conditions on $(Y,\Sigma_Y)$ than being countably separatedness, e.g. $(Y,\Sigma_Y)$ being allowed to be only separated or $\Sigma_Y$ containing all singletons $\{y\}$ for $y \in Y$.

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Packo
  • 285
  • 1
  • 6

Sufficient condition for the graph of a measurable map to be measurable

Let $f:X \to Y$ be measurable map between measurable spaces w.r.t. to their corresponding $\sigma$-algebras $\Sigma_X$ and $\Sigma_Y$, resp.

If $(X,\Sigma_X)$ is a standard Borel space can we always conclude that the graph: $$ \Gamma(f) := \{ (x,y) \in X \times Y \mid y=f(x) \} $$ is measurable, i.e. an element of the product $\sigma$-algebra $\Sigma_X \otimes \Sigma_Y$?

Note that I'm interested in the case where $(Y,\Sigma_Y)$ is a general measurable space and not restricted to e.g. being countably separated, which would imply the claim (even for general $(X,\Sigma_X)$).