4.6.1. *Proof*. Let $H_X$ and $H_Y$ be X module and $Y$ module respectively. The more usual definition of the support of a bounded operator from $H_X$ to $H_Y$ in the literature is as follows: the support of $T$ is the complement of all those points $(y,x)\in Y\times X$ for which there exist $f,g\in C_0(X)$ such that $g(y)f(x)\neq 0$ and $gTf=0$. Show that this definition is equivalent to Definition 4.1.7.

$\newcommand{\supp}{\operatorname{supp}}$

*Idea*. Take $X=Y$ and $T=\operatorname{id}$.

*Proof*. Let $U=\supp f:=\{x\in X| f(x)\neq 0\}$ and $V=\supp g$ respectively, then $U$ and $V$ are open subsets of $X$ and $Y$. Moreover,

\begin{align*}

& \chi_VT\chi_U=\frac{\chi_V}{g}gTf\frac{\chi_U}{f}\\

& gTf=g\chi_VT\chi_U f,

\end{align*}

where $\frac{\chi_U}{f}:=

\begin{cases}

\frac{1}{f(x)}, & x\in U\\

0, & \mbox{others}

\end{cases}.$

*Remark*. $\chi_{\supp f}$ is a projection in $B(H_X)$ whose range is the norm closure of $fH_X$.