Łenski Włodzimierz: On the generalization of strong summability of double Fourier series. In: Acta scientiarum mathematicarum, (60) 3-4. pp. 515-526. (1995)
Łenski Włodzimierz: On strong approximation by logarithmic means of Fourier series. In: Acta scientiarum mathematicarum, (48). pp. 285-295. (1985)