Fej\'er means of Vilenkin-Fourier series
  TThe main aim of this paper is to prove that there exist a martingale $f\in H_{1/2}$ such that Fej\'er means of Vilenkin-Fourier series of the martingale $f$ is not uniformly bounded in the space $L_{1/2}.$ 
George Tephnadze
math.CA math.FA