On the Lazarev-Lieb Extension of the Hobby-Rice Theorem

Abstract

O. Lazarev and E. H. Lieb proved that given $f_1,...,f_n\in L^1([0,1];\mathbb{C})$, there exists a smooth function $\Phi$ that takes values on the unit circle and annihilates ${span}\{f_{1},...,f_{n}}$. We give an alternative proof of that fact that also shows the $W^{1,1}$ norm of $\Phi$ can be bounded by $5\pi n+1$. Answering a question raised by Lazarev and Lieb, we show that if $p>1$ then there is no bound for the $W^{1,p}$ norm of any such multiplier in terms of the norms of $f_1,...,f_n$.