# Square integrable question

1. Oct 24, 2006

### Logarythmic

If

$$\frac{f(x)}{x}$$

is square integrable, why does

$$\int_a^b \frac{f(x)}{x} \sin{nx} dx \longrightarrow 0$$

when

$$n \rightarrow \infty$$

?

Last edited: Oct 24, 2006
2. Oct 24, 2006

### OlderDan

This is not an answer, but maybe a hint is contained in here. I think there is some theorem that will let you prove a square integrable function on a finite interval is absolutely integrable on that interval, and that demands a certain smoothness of the function. The integral of any smooth function multiplied by an infinite frequency sinusoid must be zero.