referenced not cited:
%  \bibitem{1} B. Alspach, T. C. Brown, and P. Hell, On the density of sets containing no $k$-element arithmetic progressions of a certain kind, \emph{J. London Math. Soc. (2)} \textbf{13} (1976), 226--234.
%  \bibitem{2} T. C. Brown, Behrend's theorem for sequences containing no $k$-element arithmetic progression of a certain type, \emph{J. Combin. Theory Ser. A} \textbf{18} (1975), 352--356.