G. Karagulyan and K. Muradyan, “ON THE DIVERGENCE OF WALSH AND HAAR SERIES BY SECTORIAL AND TRIANGULAR REGIONS”, Proc. YSU A: Phys. Math. Sci., vol. 48, no. 2 (234), pp. 3–12, Jul. 2014, doi: 10.46991/PYSUA.2014.48.2.003.