A recursive construction for perfect hash families

A known recursive construction for perfect hash families multiplies the number of factors by q, where q is the largest prime power not exceeding the number of symbols. This is generalized to permit multiplication of the number of factors by powers of q; it is also improved upon by restricting the structure of the ingredient designs. For a perfect hash family of strength t, it may be possible to order its rows so that, for each ℓ ≤ t, the first Σ _i=2 ^ℓ N _i rows form a perfect hash family of strength ℓ. This 'matroshka' ordering underlies a sometimes substantial reduction in the number of rows generated by the recursive construction. Perfect hash families with useful matroshka orderings are generated using linear orthogonal arrays, and with an effective one-row-at-a-time greedy algorithm.