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Designs, Codes and Cryptography

, Volume 85, Issue 3, pp 425–436 | Cite as

On symmetric BIBDs with the same 3-concurrence

  • Zongchen Chen
  • Da Zhao
Article
  • 253 Downloads

Abstract

In a symmetric balanced incomplete block design (SBIBD), every pair of points appears \(\lambda \) times among all blocks. In this paper, we study the 3-concurrence of an SBIBD, i.e., the number of times that each triple of points appears. We try to find two distinct SBIBDs whose 3-concurrences are exactly the same. Existence of such a pair would give a non-trivial tight relative 3-design on two shells in the binary Hamming association scheme H(n, 2). We prove that such pairs of designs do not exist when \(\lambda =1,2\) or the block size is at least \((\lambda -1)(\lambda ^2-2)+2\). We also give criteria to check the existence of such pairs when the designs are given. For \(\lambda =3\), our computational results show the non-existence of such pairs with only two cases left unknown: (45,12,3) and (71,15,3).

Keywords

Symmetric BIBD 3-Concurrence Johnson graph 

Mathematics Subject Classification

05B05 

Notes

Acknowledgements

We deeply thank Eiichi Bannai for suggesting the topic of this paper and for his advice and comments at various stages of this project. We thank Yaokun Wu for improving the presentation of the paper. We also thank Etsuko Bannai, Takuya Ikuta, Kyoungtark Kim and Yan Zhu for useful discussions. We are grateful to Ted Spence for the data of designs on his homepage, and especially for providing us the incidence matrices of some SBIBDs. This research was supported by NSFC Grant 11271257 and 11271255.

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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.School of MathematicsGeorgia Institute of TechnologyAtlantaUSA
  2. 2.School of Mathematical SciencesShanghai Jiao Tong UniversityShanghaiChina

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