prove that a measurable function is in $L^{\infty}([0,1])$ I am trying to solve this problem but I can not find a good idea:
Let $\phi$ be a measurable function on $[0,1]$ and assume that the linear transformation 
A $\colon f \to f \cdot \phi$ 
maps $L^{2}([0,1])$ into itself .Prove that $\phi \in L^{\infty}([0,1])$.
I tried to prove it by contradiction by choosing properly a function f so that $f\phi$ is no longer in $L^2([0,1])$ but I was no able to find any of it. Any hint for this problem would be appreciated.

The condition is that $$\int_{[0,1]}|\varphi|^2\,|f|^2<\infty$$ for all $f\in L^2$. So the operator $A_0:f\longmapsto |\varphi|\,f$ also maps $L^2$ into $L^2$. 
We also have 
$$
\int_{[0,1]}(1+|\varphi|)^2\,|f|^2\leq 2\int_{[0,1]}(1+|\varphi|^2)\,|f|^2\leq\int_{[0,1]}|f|^2+\int_{[0,1]}|\varphi|^2\,|f|^2<\infty,
$$
so the operator $T:f\longmapsto (1+|\varphi|)f$ also maps $L^2$ into $L^2$. If we define $S:f\longmapsto \tfrac1{1+|\varphi|}\,f$, then 
$$
\|Sf\|_2^2=\int_{[0,1]}|Sf|^2\leq\int_{[0,1]}|f|^2=\|f\|_2^2,
$$
so $S$ is bounded. As $TS=ST=I$, we have the $S$ is bounded and bijective. Then $T$ is bounded by the Bounded Inverse Theorem (that is, because $S$ is bounded it is open by the Open Mapping Theorem, and that makes $T$ continuous). Knowing that $T$ is bounded, we get that $A_0=T-I$ is bounded. Since
$$
\|Af\|_2^2=\int_{[0,1]}|\varphi|^2\,|f|^2=\|(A_0-I)f\|^2_2
$$
for all $f$, we get that $\|A\|=\|A_0-I\|$. 
Now let $L=\{|\varphi|>\|A\|+1\}$. Then, if $m(L)>0$, 
\begin{align}
\|A\|^2\,m(L)&=\int_{[0,1]}\|A\|^2\,1_L^2\leq\int_{[0,1]}(|\varphi|-1)^2\,1_L^2<\int_{[0,1]}|\varphi|^2\,1_L^2\\[0.3cm]&=\|A\,1_L\|_2^2\leq\|A\|^2\,\|1_L\|_2^2=\|A\|^2\,m(L).
\end{align}
a contradiction. So $m(L)=0$, making $\varphi$ essentially bounded. You can actually do the above with $\delta>0$ instead of $1$, to end up showing that $\|A\|=\|\varphi\|_\infty$. 
