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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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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. – fedja Nov 2 '12 at 12:47
@fedja, you meant Weyl's... – Asaf Nov 2 '12 at 12:52
Right! $\\ \ \ $ – fedja Nov 2 '12 at 12:59
For the general (and correct) statement you can find a quick proof here:… – GH from MO Nov 2 '12 at 18:38

Just google Kronecker's approximation theorem.

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