We prove the existence of a cyclic (4
p, 4, 1)-BIBD—and hence, equivalently, that of a cyclic (4, 1)-GDD of type 4
p
—for any prime
such that (
p–1)/6 has a prime factor
q not greater than 19. This was known only for
q=2, i.e., for
. In this case an
explicit construction was given for
. Here, such an explicit construction is also realized for
.We also give a strong indication about the existence of a cyclic (4
p 4, 1)-BIBD for
any prime
,
p>7. The existence is guaranteed for
p>(2
q
3–3
q
2+1)
2+3
q
2 where
q is the least prime factor of (
p–1)/6.Finally, we prove, giving explicit constructions, the existence of a cyclic (4, 1)-GDD of type 6
p
for any prime
p>5 and the existence of a cyclic (4, 1)-GDD of type 8
p
for any prime
. The result on GDD's with group size 6 was already known but our proof is new and very easy.All the above results may be translated in terms of optimal optical orthogonal codes of weight four with =1.
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