Talk:Operator

I hesitate to write $A:X\to Y$ instead of $A:D\to Y$ when $D \ne X$ (even though I do not hesitate to say that $A$ is an operator from $X$ to $Y$). Do others write it this way? --Boris Tsirelson (talk) 10:40, 20 December 2012 (CET)
It was in the original article " from $X$ into $Y$ ". On the other hand I agree with you and I think that $A:D\to Y$, where $D\subset X$ would be more appropriate now Nikita2 (talk) 16:46, 20 December 2012 (CET)