Convergence of integrals given pointwise convergence and some other conditions but not $L^1$ dominance.

68 Views Asked by At

Suppose

  1. $f_n, f : \mathbb{R}^d \to \mathbb{C}$ are bounded, uniformly continuous, and go to zero at infinity.

  2. $|f_n| \leq |f|$

  3. $f_n \to f$ pointwise

  4. $0 \leq \int f_n \leq 1$

  5. $\int f_n$ converges

  6. $\int |f_n| \to \int |f|$

Can we conclude $\int f_n \to \int f$?

Note that $\int |f| < \infty$ is not assumed, so it is not obvious if Fatou or dominated convergence can be applied here.

1

There are 1 best solutions below

2
On BEST ANSWER

For $n\in\mathbb N$ let $$ f_n(x) = \begin{cases} \frac{\sin(x) }{k}, & (k-1)\pi \le |x| \le k\pi, k=1,\dotsc, n, \\ 0, & \text{otherwise.} \end{cases}$$ It is easily to see that:

  1. $f_n$ has compact support, and admits the Lipschitz constant $1$.

  2. $f_n$ converges uniformly to $f$, defined by $$ f(x) = \frac{\sin(x) }{k} $$ for $(k-1)\pi \le |x| \le k\pi, k\in\mathbb N$. In particular, $f(x)\to 0$ as $|x|\to 0$ and also admits the Lipschitz constant $1$.

  3. $|f_n|$ increases to $|f|$.

  4. $\int f_n = 0$ and thus converges

  5. $\int |f_n| = 4 \sum_{k=1}^n k^{-1} \to \infty = \int |f|$.

  6. As $\int |f| = \infty$, $\int f$ isn't well-defined.

N.B. There is a result stating: If $f_n$ and $f$ are integrable and $f_n\to f$ a.e. Then, $f_n \to f$ in $L^1$ if and only if $\int |f_n| \to \int |f|$.