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### Lacunary sequence

Is there a standard definition for a lacunary sequence?
Suppose $0 < a_1 < a_2 < \cdots.$
I've read two papers using the term recently. One requires
$$
\liminf_n\frac{a_{n+1}}{a_n}>1
$$
...

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### Coefficients of lacunary series on quasiconformally transformed unit disk

Say I have a lacunary $q$ series $s(q)=\sum_{n=0}^{\infty} a_{n}q^{n}$ , and I have a quasiconformal transformation $\xi$ which preserves the boundary of the unit disk in $\mathbb{C}$ such that if ...