1. A. Aasma, H. Dutta, P.N. Natarajan, An Introductory Course in Summability Theory, JohnWiley and Sons, Hoboken, USA, 2017.
2. J. Boos, Classical and Modern Methods in Summability, University Press, Oxford, 2000.
3. G. Kangro, On the summability factors of the Bohr-Hardy type for a given speed I., Proc. Estonian Acad. Sci. Phys. Math. 18 (1969) 137-146 (in Russian).
4. G. Kangro, On the summability factors of the Bohr-Hardy type for a given speed II)., Eesti NSV Tead. Akad. Toimetised 18(4) (1969) 387-395.
5. G. Kangro, Summability factors for the series λ-bounded by the methods of Riesz and Cesàro, Acta Comment. Univ. Tartuensis 277 (1971) 136-154 (in Russian).