Krdytkyu

Let $(X,Sigma)$ be a measurable space, and let $Y$ be a topological space.
Let $f_n:X to Y$ be a measurable sequence, when we take on $Y$ the Borel sigma algebra. In other words the $f_n$ are measurable as mappings between the measurable spaces
$$ (X,Sigma) to (Y,B(Y)).$$

Is $E= { xin X : f_n(x) text{ is convergent in } Y }$ measurable?

This is well-known (and easy) in the case of real-valued functions. Indeed,

$$ E=bigcap_{kinmathbb{N}}bigcup_{ninmathbb{N}}bigcap_{r,s >n}left{ xin X:left|f_{r}left(xright)-f_{s}left(xright)right|<frac{1}{k}right}. $$

I think this proof could be adapted to the case where $Y$ is a complete metric space; just replace $$ left|f_{r}left(xright)-f_{s}left(xright)right|$$ with
$$ dleft(f_{r}(x),f_{s}(x)right) .$$

Does this hold for non-complete metric spaces as targets? What about when $Y$ is not a metric space?

This is a really instructive question: apparently, completeness is the key!

First, let $X=Y=[0,1]$ with the usual topology. We define an example for a sequence of functions $f_n: Xrightarrow Y$, which converges everywhere point-wise. Then we ruin $Y$.

Let $f_1$ be constant $0$ on $[0,1/2[$ and constant $1/2$ on $[1/2,1]$.
Let $f_2$ be constant $0$ on $[0,1/4[$, constant $1/4$ on $[1/4,1/2[$, constant $1/2$ on $[1/2,3/4[$, and constant $3/4$ on $[3/4,1]$.
In general, divide $[0,1]$ into $2^n$ intervals of equal length (closed from the left and open from the right, except for the last one which is closed), and let $f_n$ be the step function whose value on each interval is the left endpoint.
It is clear that this sequence of functions converges point-wise to the identity function on $[0,1]$.

We keep these functions, but modify $Y$ now. Pick any non-measurable set $Ssubseteq [0,1]$, and let $D$ consist of those rational numbers in $[0,1]$ whose denominator is a power of $2$.
Let $Y$ be the topological subspace of $[0,1]$ induced by $Scup D$. This is of course a metric space (as it is a subspace of a metric space), but it is no longer complete!

Define the same sequence of functions as above; they are still measurable (and also functions $Xrightarrow Y$, because we were careful enough to put all elements of $D$ in $Y$).
However, the domain of convergence is $Y= Scup D$: a sequence in $Y$ is convergent if and only if it is convergent as a sequence in $[0,1]$ with limit in $Y$.
As $S$ is not measurable and $D$ is countable, we have that $Y$ is not measurable (as a subset of $X$).