@inbook{10.4169/j.ctt5hh8v6.13,
ISBN = {9780883850473},
URL = {http://www.jstor.org/stable/10.4169/j.ctt5hh8v6.13},
abstract = {Let$D = \{ {d_1},{d_2},.,{d_k}\} $be ak-set of integers modulovsuch that every${\rm A} \ne 0$(modv) can be expressed in exactly λ ways in the form(1.1)${d_i} - {d_j} \equiv a(\bmod v)$,where${d_i}$and${d_j}$are inD. We suppose further that(1.2)$0 < \lambda < k < v - 1$.The inequality (1.2) serves only to exclude certain degenerate configurations. A setDfulfilling these requirements is called aperfect difference setor, for brevity, adifference set.It is easy to verify that(1.3)$\lambda = \frac{{k(k - 1)}} {{v - 1}}$.This assertion is also an immediate consequence of the following theorem.THEOREM 1.1.The perfect difference set D is equivalent to$a(v,k,\lambda )$-configuration with incidence},
author = {HERBERT JOHN RYSER},
booktitle = {Combinatorial Mathematics},
edition = {1},
pages = {131--142},
publisher = {Mathematical Association of America},
title = {PERFECT DIFFERENCE SETS},
volume = {14},
year = {1963}
}