## Tanja Eisner, Michael Lin, On modulated ergodic theorems

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DOI: 10.23952/jnva.2.2018.2.03
Volume 2, Issue 2, 1 August 2018, Pages 131-154

Abstract. Let $T$ be a weakly almost periodic (WAP) linear operator on a Banach space $X$. A sequence of scalars $(a_n)_{n\geq 1}$ modulates $T$ on $Y\subset X$ if $\frac{1}{n}\sum_{k=1}^n a_kT^k x$ converges in norm for every $x \in Y$. We obtain a sufficient condition for $(a_n)$ to modulate every WAP operator on the space of its flight vectors, a necessary and sufficient condition for (weakly) modulating every WAP operator $T$ on the space of its (weakly) stable vectors, and sufficient conditions for modulating every contraction on a Hilbert space on the space of its weakly stable vectors. We study as an example modulation by the modified von Mangoldt function $\Lambda'(n):=\log n1_\mathbb P(n)$ (where $\mathbb P =(p_k)_{k\ge 1}$ is the sequence of primes), and show that, as in the scalar case, convergence of the corresponding modulated averages is equivalent to convergence of the averages along the primes $\frac{1}{n}\sum_{k=1}^n T^{p_k}x$. We then prove that for any contraction $T$ on a Hilbert space $H$ and $x \in H$, and also for every invertible $T$ with $\sup_{n \in \mathbb Z} \|T^n\| <\infty$ on $L^r(\Omega,\mu)$ ($1) and $f \in L^r$, the averages along the primes converge.