相邻 $k$-free 整数之间的间隙
On the gaps between consecutive $k$-free integers
H. Halberstam & K. F. Roth
http://jlms.oxfordjournals.org/content/s1-26/4/268.extract
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J. London Math. Soc. (1951) s1-26 (4): 268-273. doi: 10.1112/jlms/s1-26.4.268
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