As the title says, I'm trying to find the completeness radius of $\{2,3,5,7,11,\ldots\}$. The completeness radius of a sequence $\Lambda=\{\lambda_n\}$ is $R(\Lambda)=\sup\{A~|~\{e^{i\lambda_n t}\text{ is complete in }C[-A,A]\}$, where completeness in this case means that any $f\in C[-A,A]$ can be uniformly approximated by a finite linear combination of the $\{e^{i\lambda_n t}\}$. I've already shown that the density of this sequence is 0.
This problem appears in the book Introduction to Nonharmonic Fourier Series by Robert M. Young. I'm trying to work through the book, but I have a hard time understanding the ideas in a lot of his exercises. If you know where I can find solutions for exercises in that book, please let me know.