Lal, S., and H. K. Nigam. “Degree of Approximation by $ (N, P, Q) $ Summability Means of the Fourier-Laguerre Expansion”. Tamkang Journal of Mathematics, vol. 32, no. 2, June 2001, pp. 143-50, doi:10.5556/j.tkjm.32.2001.357.