Bruck-Chowla-Ryser theorem
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The Bruck-Chowla-Ryser theorem is a result on the combinatorics of block designs. It states that if a (v; k; λ) design exists, then:
- k − λ is a square,
when v is even; and the diophantine equation
- x2 − (k − λ)y2 − (−1)(v−1)/2 λ z2 = 0
has a nontrivial solution, when v is odd.