A class of real cocycles over an irrational rotation for which Rokhlin cocycle extensions have Lebesgue component in the spectrum

Magdalena Wysokińska


We describe a class of functions $f\colon {\mathcal B}/{\mathbb Z} \to {\mathcal B}$ such that
for each irrational rotation $Tx=x+\alpha$, where $\alpha$ has the
property that the sequence of aritmethical means of its
partial quotients is bounded, the corresponding weighted unitary operators
$L^2({\mathcal B}/{\mathbb Z})\ni g \mapsto e^{2\pi i c f}\cdot g\circ T$
have a Lebesgue spectrum for each $c\in {\mathbb R}\setminus\{0\}$.
We show that for such $f$ and $T$ and for an
arbitrary ergodic ${\mathcal B}$-action ${\mathcal S}=(S_t)_{t\in {\mathcal B}}$ on $(Y,{\mathcal C},\nu)$
the corresponding
Rokhlin cocycle extension $T_{f,{\mathcal S}}(x,y)=(Tx,S_{f(x)}y)$ acting
on $({\mathcal B}/{\mathbb Z}\times Y,\mu \otimes \nu)$ has also a Lebesgue spectrum in
the orthogonal complement of $L^2({\mathcal B}/{\mathbb Z},\mu)$
and moreover the weak closure of powers of $T_{f,{\mathcal S}}$ in the space of
self-joinings consists of ergodic elements.


Ergodicity; Rokhlin cocycle extension; Lebesgue spectrum; mixing; joining; ELF

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