[1] C. R. Adams, On summability of double series, Trans. Amer. Math. Soc. 34(2) (1932) 215–230.
[2] C. R. Adams, On non-factorable transformations of double sequences, Proc. Natl. Acad. Sci. USA 19 (1933) 564–567.
[3] N. A. Bokayev and Zh. B. Mukanov, Weighted integrability of double trigonometric series and of double series with respect to multiplicative systems with coefficients of class R+0 BV S2, Translation of Mat. Zametki 91 (2012), no. 4, 617-620. Math. Notes 91 (2012), no. 3-4, 575–578.
[4] P. Chandra, On the degree of approximation of functions belonging to the Lipschitz class, Nanta Math. 8(1) (1975) 88–91.
[5] P. Chandra, On the degree of approximation of a class of functions by means of Fourier series, Acta Math. Hungar. 52 (1988) 199–205.
[6] P. Chandra, A note on the degree of approximation of continuous functions, Acta Math. Hungar. 62 (1993) 21–23.
[7] Y. S. Chow, On the Ces`aro summability of double Fourier series, Tohoku Math. J. 5 (1953) 277–283.
[8] J. G. Herriot, N¨orlund summability of double Fourier series, Trans. Amer. Math. Soc. 52 (1942) 72–94.
[9] E. Hille and J. D. Tamarkin, On the summability of Fourier series, Trans. Amer. Math. Soc. 32 (1932) 757–783.
[10] A. S. B. Holland, B. N. Sahney, J. Tzimbalario, On degree of approximation of a class of functions by means of Fourier series, Acta Sci. Math. (Szeged) 38(1-2) (1976) 69–72.
[11] Xh. Z. Krasniqi, On the degree of approximation of continuous functions that pertains to the sequence-to-sequence transformation, Aust. J. Math. Anal. Appl. 7(2) (2011) 1–10.
[12] Xh. Z. Krasniqi, On the degree of approximation of continuous functions by matrix means related to partial sums of a Fourier series, Comment. Math. 52(2) (2012) 207–215.
[13] Xh. Z. Krasniqi, Approximation of continuous functions by generalized deferred Voronoi-N¨orlund means of partial sums of their Fourier series, An. S¸tiint¸. Univ. Al. I. Cuza Ia¸si. Mat. (N.S.) 66(1) (2020) 37–53.
[14] Xh. Z. Krasniqi, On the degree of approximation of continuous functions by a specific transform of partial sums of their Fourier series, Acta Comment. Univer. Tartuensis de Math. (accepted)
[15] Xh. Z. Krasniqi, On the degree of approximation of conjugate functions of periodic continuous functions, Poincare J Anal. Appl. 7(2) (2020) 175–184.
[16] Xh. Z. Krasniqi, On the degree of approximation of continuous functions by a linear transformation of their Fourier series, Commun. Math. (accepted).
[17] Xh. Z. Krasniqi, Applications of the deferred generalized de la Vall´ee Poussin means in approximation of continuous functions, Studia Univ. Babe¸s-Bolyai Math.(accepted).
[18] S. Lal and V. N. Tripathi, On the study of double Fourier series by double matrix summability method, Tamkang J. Math. 34(1) (2003) 1–16.
[19] S. Lal and H. P. Singh, Double matrix summability of double Fourier series, Int. J. Math. Anal. (Ruse) 3(33-36) (2009) 1669–1681.
[20] L. Leindler, On the uniform convergence and boundedness of a certain class of sine series, Anal. Math. 27(4) (2001) 279–285.
[21] L. Leindler, On the degree of approximation of continuous functions, Acta Math. Hungar. 104 (2004) 105–113.
[22] V. N. Mishra, S. K. Paikray, P. Palo,; P. N. Samanta, M. Misra, U. K. Misra, On double absolute factorable matrix summability, Tbilisi Math. J. 10(4) (2017) 29–44.
[23] N. L. Mittal and B. E. Rhoades, Approximation by matrix means of double Fourier series to continuous functions in two variables, Rad. Mat. 9(1) (1999) 77–99.
[24] F. M´oricz and B. E. Rhoades, Approximation by N¨orlund means of double Fourier series to continuous functions in two variables, Constr. Approx. 3(3) (1987) 281–296.
[25] F. M´oricz and X. L. Shi, Approximation to continuous functions by Ces`aro means of double Fourier series and conjugate series, J. Approx. Theory 49(4) (1987) 346–377.
[26] F. M´oricz and B. E. Rhoades, Approximation by N¨orlund means of double Fourier series for Lipschitz functions, J. Approx. Theory 50(4) (1987) 341–358.
[27] J. N´emeth, A remark on the degree of approximation of continuous functions, Acta Math. Hungar. 106(1-2) (2005) 83–88.
[28] A. Rathore, U. Singh, Approximation of certain bivariate functions by almost Euler means of double Fourier series, J. Inequal. Appl. 2018, Paper No. 89, 15 pp.
[29] B. E. Rhoades, On absolute normal double matrix summability methods, Glas. Mat. Ser. III 38(58) (2003) 57–73.
[30] M. G. Robison, Divergent double sequences and series, Trans. Amer. Math. Soc. 28(1) (1926) 50–73.
[31] B. N. Sahney and D. S. Goel, On the degree of approximation of continuous functions, Ranchi Univ. Math. J. 4 (1973) 50–53.
[32] S¸. Sezgek and ˙I. Da˘gadur, Approximation by double Ces`aro submethods of double Fourier series for Lipschitz fuctions, Palest. J. Math. 8(1) (2019) 71–85.
[33] P. L. Sharma, On the harmonic summability of double Fourier series, Proc. Amer. Math. Soc. 91 (1958) 979–986.
[34] B. Wei and D. Yu, On the degree of approximation of continuous functions by means of Fourier series, Math. Commun. 17 (2012) 211–219.
[35] Y. Zhao and D. Yu, Approximation by T-transformation of double Walsh-Fourier series to multivariable functions, ISRN Math. Anal. 2014, Art. ID 713175, 14 pp.