Bruck-Ryser-Chowla theorem
properNoun
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(math) A result on the combinatorics of block designs, stating that, if a (v, b, r, k, λ)-design exists with v = b (a symmetric block design), then: (i) if v is even, then k − λ is a square; (ii) if v is odd, then the following Diophantine equation has a nontrivial solution: x2 − (k − λ)y2 − (−1)(v−1)/2 λ z2 = 0.