Erdős, Paul, On the Representation of an Integer as the Sum of k k-th Powers. J. London Math. Soc. (1936), 133-136.

Erdős, Paul, Some recent advances and current problems in number theory. Lectures on Modern Mathematics, Vol. III (1965), 196-244.

Erdős, P. and Graham, R., Old and new problems and results in combinatorial number theory. Monographies de L'Enseignement Mathematique (1980). Library home: Erdős and Graham 1980.

Guy, Richard K., Unsolved problems in number theory. 3rd ed., Problem Books in Mathematics, Springer, New York (2004), xviii+437 pp. Section D4 "Waring's problem. Sums of $l$ $k$th Powers.", printed p. 229: Mahler's disproof of Hypothesis K for $k=3$, "Erdős thinks it possible that for all $n$, $r_{3,3}<c_2n^{1/12}$ but nothing is known", and the Chowla--Erdős bound $r_{k,k}>\exp(c_k\ln n/\ln\ln n)$ for infinitely many $n$; the page's question is not stated there. Library home: Guy 2004.

Mahler, Kurt, Note on Hypothesis K of Hardy and Littlewood. J. London Math. Soc. (1936), 136-138.