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Combinatorics

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Incidence Matrix and its Transpose

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Let $A$ be the incidence matrix of a $(b,v,5,k,3)$ BIBD.

Then:

$$AA^{T}=aI+bJ$$

...where $I$ is the identify matrix and $J$ is the all $1$'s matrix.

What are the values of $a$ and $b$?