@article{Łenski:2018,
  author  = "W. Łenski and B. Szal",
  title   = "Pointwise approximation of 2\pi/r-periodic functions by matrix operators of their Fourier series with r-differences of the entries",
  journal = "Demonstratio Mathematica",
  volume  = "Vol. 51",
  number  = "iss. 1",
  pages   = "309--322",
  year    = "2018",
}