\bib{1951/debruijn-1}
\yr 1951
\mr 13,724e
\by Nicolaas G. de Bruijn
\paper On the number of positive integers $\le x$ and free of prime factors $>y$
\jour Indagationes Mathematicae
\issn 0019--3577
\vol 13
\pages 50--60
\endref