# Thread: example, sequence of measurable

1. ## example, sequence of measurable

Give an example in $\displaystyle [0, 1]$ of a sequence of measurable functions $\displaystyle (f_n)_n$ such that $\displaystyle \lim_{n \rightarrow \infty } || f_n ||_1=0$ but $\displaystyle (f_n)_n$ does not converge to zero a.e.

For this one, I can't think of a sequence that fits this criteria. What would this sequence look like? I can't think of one right now.

2. Originally Posted by xianghu21
Give an example in $\displaystyle [0, 1]$ of a sequence of measurable functions $\displaystyle (f_n)_n$ such that $\displaystyle \lim_{n \rightarrow \infty } || f_n ||_1=0$ but $\displaystyle (f_n)_n$ does not converge to zero a.e.
Have a look at this:

Choose a sequence $\displaystyle (a_n)_n$ in $\displaystyle [0,1]$ and real numbers $\displaystyle r_n\to_n 0$ such that every $\displaystyle x\in[0,1]$ belong to infinitely many intervals $\displaystyle [a_n,a_n+r_n]$.

For instance, $\displaystyle a_0=0, a_1=\frac{1}{2}, a_2=0, a_3 =\frac{1}{4}, a_4=\frac{1}{2}, a_5=\frac{3}{4}, a_6=0$, etc. (dyadic points) and $\displaystyle r_0=r_1=\frac{1}{2}, r_2=r_3=r_4=r_5=\frac{1}{4}, r_6=\cdots = r_{13} = \frac{1}{8}$,...

Then let $\displaystyle f_n(x)={\bf 1}_{[a_n,a_n+r_n]}(x)$.

There may be simpler, that's what came to my mind. Since $\displaystyle (f_n)_n$ can't be monotonic, it must look like my example anyway.