Approximation of Signals (Functions) by Trigonometric Polynomials in -Norm

Mittal and Rhoades (1999, 2000) and Mittal et al. (2011) have initiated a study of error estimates through trigonometric-Fourier approximation (tfa) for the situations in which the summability matrix does not have monotone rows. In this paper, the first author continues the work in the direction for to be a -matrix. We extend two theorems on summability matrix of Deger et al. (2012) where they have extended two theorems of Chandra (2002) using -method obtained by deleting a set of rows from Cesàro matrix . Our theorems also generalize two theorems of Leindler (2005) to -matrix which in turn generalize the result of Chandra (2002) and Quade (1937). “In memory of Professor K. V. Mital, 1918 - 2010.” 1. Introduction Let be a periodic signal (function) and let . Let denote the partial sums, called trigonometric polynomials of degree (or order) , of the first terms of the Fourier series of at a point . The integral modulus of continuity of is defined by If, for , then . Throughout will denote the -norm, defined by A positive sequence is called almost monotone decreasing (increasing) if there exists a constant , depending on the sequence only, such that, for all , Such sequences will be denoted by and , respectively. A sequence which is either or is called almost monotone sequence and will be denoted by . Let be an infinite subset of and as the range of strictly increasing sequence of positive integers; say . The Cesàro submethod is defined as where is a sequence of real or complex numbers. Therefore, the -method yields a subsequence of the Cesàro method , and hence it is regular for any . is obtained by deleting a set of rows from Cesàro matrix. The basic properties of -method can be found in [1, 2]. In the present paper, we will consider approximation of by trigonometric polynomials and of degree (or order) , where and by convention . The case for all of either or yields We also use Mittal and Rhoades [3, 4] have initiated the study of error estimates through trigonometric-Fourier approximation (tfa) for the situations in which the summability matrix does not have monotone rows. In this paper, the first author continues the work in the direction for to be a -matrix. Recently, Chandra [5] has proved three theorems on the trigonometric approximation using -matrix. Some of them give sharper estimates than the results proved by Quade [6], Mohapatra and Russell [7], and himself earlier [8]. These results of Chandra [5] are improved in different directions by different investigators such as Leindler [9] who dropped the monotonicity on generating sequence and