1. B. I. Golubov, A. V. Efimov, and V. A. Skvortsov, Walsh Series and Transforms: Theory and Applications (Nauka, Moscow, 1987) [in Russian].
2. F. Schipp, W. Wade, and P. Simon, Walsh Series (Akad. Kiado, Budapest, 1990).
3. M. T. Chen, “The Absolute Convergence of Fourier Series,” Duke Math. J. 9(4), 803–810 (1942).
4. M. Izumi and S.-I. Izumi, “Absolute Convergence of Fourier Series of Convolution Functions,” J. Approx. Theory 1(1), 103–109 (1968).
5. C.W. Onneweer, “On Absolutely Convergent Fourier Series,” Arkiv Mat. 12(1), 51–58 (1974).