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Given $a$ and $b$ irrational numbers with $a/b$ also irrational, how do you prove that $( \{ na\} , \{ nb \})$ is dense in $[0,1] * [0,1]$ , where $n$ ranges over the integers?

$\{x\}$ is the fractional part of $x$ .

I'm also curious about the general case, with $n$ irrational numbers , linearly independent over $Q$ , resulting in density over $[0,1]^n$ .

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    $\begingroup$ You don't. Take $a=\sqrt 2$ and $b=1+\sqrt 2$. The correct condition is that $1,a,b$ are linearly independent over $\mathbb Q$. Weil's criterion does the trick in no time giving even the equidistribution property. $\endgroup$
    – fedja
    Commented Nov 2, 2012 at 12:47
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    $\begingroup$ @fedja, you meant Weyl's... $\endgroup$
    – Asaf
    Commented Nov 2, 2012 at 12:52
  • $\begingroup$ Right! $\\ \ \ $ $\endgroup$
    – fedja
    Commented Nov 2, 2012 at 12:59
  • $\begingroup$ For the general (and correct) statement you can find a quick proof here: mathoverflow.net/questions/106819/… $\endgroup$
    – GH from MO
    Commented Nov 2, 2012 at 18:38

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Just google Kronecker's approximation theorem.

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