## Multiplicative zero-one laws and metric number theory.(English)Zbl 1292.11085

The authors study multiplicative analogues of the celebrated Duffin–Schaeffer conjecture [R. J. Duffin and A. C. Schaeffer, Duke Math. J. 8, 243–255 (1941; Zbl 0025.11002)] in metric diophantine approximation. To be precise, for a function $$\psi: {\mathbb N} \rightarrow [0,1/2)$$, they study the set $${\mathcal S}^\times_n (\psi)$$ consisting of the points $$(x_1, \dots, x_n) \in [0,1]^n$$ for which the inequality $\prod_{i=1}^n \| q x_i \| < \psi(q)$ has infinitely many solutions $$q \in {\mathbb N}$$, where $$\| \cdot \|$$ denotes the distance to the nearest integer. Additionally, they study the set $${\mathcal D}^\times_n (\psi)$$, in which the $$\| qx_i \|$$ is replaced with the distance to the nearest integer co-prime to $$q$$.
It is shown that in both cases, the sets satisfy a zero-one law, i.e. the Lebesgue measure of either set must be equal to zero or one. Additionally, a criterion under which the measure must be equal to one is derived in the spirit of the Duffin–Schaeffer theorem [loc. cit.]. The conditions are fairly technical, and the authors state conjectures giving a simpler necessary and sufficient conditions for the measure to be equal to one. Finally, the second main result is applied to obtain a metrical result related to the $$p$$-adic Littlewood conjecture. At the heart of the proofs is a ‘cross-fibering technique’, allowing one to reverse the implications in Fubini’s theorem.

### MSC:

 11J83 Metric theory 11J13 Simultaneous homogeneous approximation, linear forms 11K60 Diophantine approximation in probabilistic number theory

Zbl 0025.11002
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