I'm trying to read this paper: <cite authors="Bourgain, J.; Jitomirskaya, S.">_Bourgain, J.; Jitomirskaya, S._, [**Continuity of the Lyapunov exponent for quasiperiodic operators with analytic potential**](https://doi.org/10.1023/A:1019751801035), J. Stat. Phys. 108, No. 5-6, 1203-1218 (2002), [arXiv:math-ph/0110040](https://doi.org/10.48550/arXiv.math-ph/0110040). [Zbl&nbsp;1039.81019](https://zbmath.org/1039.81019). [CiteSeerX](https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.20.1813)</cite>

But I don't understand the norm they are using in (1.3). They write down the condition
$\vert\vert k\omega\vert\vert>C(\vert k\vert \log(1+\vert k\vert)^A)^{-1}$ 

($\omega$ is an irrational number, and I think $k$ is an integer), and say that this expression is a "strong Diophantine condition" on $\omega$.

Another line in the paper which may be illuminating is 

"for $\vert k\vert\leq q/2, \vert kw-ka/q\vert<1/2q$ and hence $\vert\vert k\omega\vert\vert >1/2q$." 

Here $a/q$ is a lowest terms fraction with property $\vert \omega-a/q\vert<1/q^2$.

 Context suggests to me that the norm should be a related to how easy it is to approximate an irrational number with rationals, but they don't have a definition in the paper, and I don't think I understand Diophantine approximation well enough to guess what it is.