Abstract
A strong indication about the existence of a (7p, 4, 1) difference family with p ≡ 7 (mod 12) a prime has been given in [11]. Here, developing some ideas of that paper, we give, much more generally, a strong indication about the existence of a cyclic (pq, 4, 1) difference family whenever p and q are primes congruent to 7 (mod 12) and of a cyclic (pq, 5, 1) difference family whenever p and q are primes congruent to 11 (mod 20). Indeed we give an algorithm for their construction that seems to be always successful and we have checked it works whenever both primes p and q do not exceed 1,000. All our (pq, 4, 1) and (pq, 5, 1) difference families have the nice property of admitting a multiplier of order 3 or 5, respectively, that fixes almost all base blocks. As an intermediate result we also find an optimal (p, 5, 1) optical orthogonal code for every prime p ≡ 11 (mod 20) not exceeding 10,000.
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Communicated by K. T. Arasu.
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Buratti, M., Pasotti, A. Further progress on difference families with block size 4 or 5. Des. Codes Cryptogr. 56, 1–20 (2010). https://doi.org/10.1007/s10623-009-9335-6
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DOI: https://doi.org/10.1007/s10623-009-9335-6