@article{1951-debruijn-1
,author={Nicolaas G. de Bruijn}
,ISSN={0019--3577}
,journal={Indagationes Mathematicae}
,MR={13,724e}
,pages={50--60}
,title={{On the number of positive integers $\le x$ and free of prime factors $>y$}}
,volume={13}
,year={1951}
}