Variational estimates for averages and truncated singular integrals   along the prime numbers
  We prove, in a unified way, $r$-variational estimates, $r>2$, on $\ell^{s}(\mathbb{Z})$ spaces, $s \in (1, \infty)$, for averages and truncated singular integrals along the set of prime numbers. 
Mariusz Mirek and Bartosz Trojan and Pavel Zorin-Kranich
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