1. A. Haar, Zur Theorie der orthogonalen Funktionensysteme, Inaugural-dissertation, GeorgAugust-Universitat, Göttingen, (1909).
2. P.L. Ulyanov, Series in Haar system, Matem. sbornik. 63(1) (1964) 356-391. (in Russian).
3. Z. Ciesielski, On Haar functions and on the Schauder Basis of the Space C⟨0, 1⟩, Bulletin de L'academie Polonaise Des Sciences. Serie des sci. math., astr. et phys. 7(4) (1959) 227-232.
4. B.I. Golubov, On Fourier series of continuous functions with respect to a Haar system, Izv. Akad. Nauk SSSR. Ser. Mat. 28 (1964) 1271-1296. (in Russian).
5. I.I. Sharapudinov, Approximation of functions in variable-exponent Lebesgue and Sobolev spaces by finite Fourier-Haar series, Sb. Math. 205(2) (2014) 291-306.