On the possible volume of $\mu$-$(v,k,t)$ trades

Abstract

A $\mu$-way $(v,k,t)$ $trade$ of volume $m$ consists of $\mu$ disjoint collections $T_1$, $T_2,\dots T_{\mu }$, each of $m$ blocks, such that for every $t$-subset of $v$-set $V$ the number of blocks containing this t-subset is the same in each $T_i(1\le i\le \mu )$. In other words any pair of collections $\{T_i,T_j\}$, $1\le i<j\le \mu$ is a $(v,k,t)$ trade of volume $m$. In this paper we investigate the existence of $\mu$-way $(v,k,t)$ trades and also we prove the existence of: (i)~3-way $(v,k,1)$ trades (Steiner trades) of each volume $m,m\ge 2$. (ii) 3-way $(v,k,2)$ trades of each volume $m,m\ge 6$ except possibly $m=7$. We establish the non-existence of 3-way $(v,3,2)$ trade of volume 7. It is shown that the volume of a 3-way $(v,k,2)$ Steiner trade is at least $2k$ for $k\ge 4$. Also the spectrum of 3-way $(v,k,2)$ Steiner trades for $k=3$ and 4 are specified.

arXiv comment12 pages, (accepted). Bull. Iranian Math. Soc., 2013