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Published online by Cambridge University Press: 20 November 2018
The purpose of this note is to give a short proof of a generalisation of a theorem of Ryser, Theorem 10.2.3 of [1], concerning matrices that occur in the theory of symmetric block designs.
The two main results of matrix theory required in the proof given below are:
(1) If B, C are square matrices such that BC = zI where z is a non-zero complex number, then CB = zI
(2) A matrix S which is both symmetric (i.e. S' = S) and skew-symmetric (i.e. S' = —S)is zero.