1. J. W. S. Cassels,An Introduction to Diophantine Approximation, Cambridge University Press, 1957.
2. I. N. Stewart and D. O. Tall,Algebraic Number Theory, Chapman and Hall (London, 1979).
3. I. Joó, On Riesz bases,Annales Univ. Sci. Budapest., Sectio Math.,31 (1988), 141–153.
4. P. Erdős and I. Joó, On the expansion $$1 = \sum {q^{ - n_i } } $$ ,Per. Math. Hung.,23 (1991).
5. P. Erdős, M. Joó and I. Joó, On a problem of Tamás Varga (to appear).