In this paper we study the sequence (ci)0id for a distance-regular graph. In particular we show that if d 2j and cj = c > 1 then c2j > c holds. Using this we give improvements on diameter bounds by Hiraki and Koolen [5], and Pyber [8], respectively, by applying this inequality.
Sejeong Bang, Akira Hiraki, Jacobus H. Koolen