On solving isomorphism problems about 2-designs using block intersection numbers

Abstract

In this paper, we give a partial solution to a new isomorphism problem about $2$-$(v,k,k-1)$ designs from disjoint difference families in finite fields and Galois rings. Our results are obtained by carefully calculating and bounding some block intersection numbers, and we give insight on the limitations of this technique. Moreover, we present results on cyclotomic numbers and on the structure of Galois rings of characteristic $p^2$.

20 pages. v2: Added additional results. Fixed a mistake in the main theorem. arXiv admin note: text overlap with arXiv:1807.02055