Let A be a bounded interval in R. Suppose we have a collection of functions, such that
Each function is $\in$ $L^r(A)$, where r is any number $\in$ $[1, \infty]$,
The fractional derivative of order 1/2 of the functions are bounded set in $L^2(A)$.
I want to know whether the set is precompact in $L^2(A)$ in the norm topology?
Actually, I was reading a paper, it seems to me that the author assumed that this is correct. I think it might be a well-know fact in distribution theory. But I do not know the reference for the statement.