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Let D be a BIBD with parameters (b,v,r,3,1). Prove that either v=6i+1 or v=6i+3 for some non-negative integer i. BIBD stands for a Balanced Incomplete Block Design

My work, theorem states that bk=vr in BIBD, which I can use to solve the problem. my basic step would be solving the equation with given v=6i+1.

Any suggestion on how to go about doing it?

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    You should not substitute anything for $v$. The question asks you to prove that $v$ must be congruent to either $1$ or $3$ modulo $6$. I think that you will find the other basic theorem tying the parameters together also very useful here.2012-04-18

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