
The authors introduce the notion of a symmetric graph design (SGD) which is a generalization of a symmetric balanced incomplete block design. In particular, a symmetric \((n,G,\lambda;F)\)-design is a collection \(\{G_1, G_2, \dots{}, G_n\}\) of spanning subgraphs of the complete graph \(K_n\) such that: (i) \(G_i \simeq G\) for \(i = 1, \dots{}, n\); (ii) every edge of \(K_n\) is contained in exactly \(\lambda\) subgraphs \(G_i\), and (iii) \(G_i \cap G_j \simeq F\) for all \(i, j, i \neq j\), and \(|E(F)|= \lambda' = {{\lambda}\choose{2}}\). The paper consists of a series of results on existence of various families of SGDs. Just some of the results will be summarized here. Trivially every symmetric \((v,k,\lambda)\)-BIBD is a \((v,K_k,\lambda)\)-SGD. A further trivial example is an \((n,K_{n-1},n-2; K_{n-2})\)-SGD which exists for all \(n\). For \(\lambda = 1\) an \((n,G,1)\)-SGD is an edge-disjoint decomposition of \(K_n\) into copies of \(G\) where \(G\) is a spanning subgraph with \((n-1)/2\) edges. When \(\lambda = 2\) an \((n,G,2)\)-SGD is an orthogonal double cover (ODC) of \(K_n\) by \(G\), and such objects have been well studied. For \(\lambda = 3\) the authors ask for which pairs \(G\), \(F\) does there exist an \((n,G,3;F)\)-SGD? Although it appears hopeless to determine the spectrum in the general case, the authors do give some results for the more restricted question where \(F \simeq K_3\). They show that if there exists an \((n,k,1)\)-BIBD then there exists an \((n,G_{r,k-1},k-2; K_{k-2})\)-SGD where \(r = (n-1)/(k-1)\), and \(G_{r,t}\) is the graph with \(rt+1\) vertices and \(r+1\) components, \(r\) of which are \(K_t\) and one is \(K_1\). As a corollary an \((n,G_{(n-1)/4,4},3;K_3)\)-SGD exists for all \(n \equiv 1,5 \pmod{20}\). But the question of whether an \((n,G_{(n-1)/4,4},3;K_3)\)-SGD exists for the remaining values of \(n \equiv 1 \pmod{4}\) remains open. By analogy with the well-known result on ODCs the authors conjecture that for all \(k \geq 4\) there exists a constant \(n_0\) such that an \((n,G_{(n-1)/(k-1),k-1},k-2; K_{k-2})\)-SGD exists for all \(n \equiv 1 \pmod{k-1}\), \(n \geq n_0\). If \(F_{r,k}\) is the connected graph with \(r(k-1)+1\) vertices consisting of \(r\) copies of \(K_k\) glued together at a single common vertex, the authors show that an \((n,F_{r,k},k;K_k)\)-SGD (where \(r = (n-1)/(k-1)\)) exists if and only if there exists an \((n,k,1)\)-BIBD. Following a PBD-closure result for rooted SGDs, the authors then establish the triplication result that if there exists an \((n,G,3;K_3)\)-SGD then there exists a \((3n,G^*,3;K_3)\)-SGD where \(G^*\) is the graph on \(3n\) vertices obtained from the \(n\)-vertex graph \(G\) by appending at each vertex a triangle with two new vertices, one of which is common to all appended triangles. This leads to the following generalization. Let \(G\) be an \(n\)-vertex graph, and suppose there exists a \((n,G,t';K_t)\)-SGD. If there exists a transversal design \(\text{TD}(2t,n)\) then there exists a \((tn,G^{(t)},t;K_t)\)-SGD. Finally the authors show that if \(n\) is a prime power and \(F\) is a graph with \({n+1}\choose{2}\) edges which has a \(\rho\)-labelling, then there exists an \((n^2+n+1,G,n+1;F)\)-SGD for some graph \(G\) with \((n+1){{n+1}\choose{2}}\) edges.
orthogonal double cover, symmetric balanced incomplete block design, symmetric graph design, Combinatorial aspects of block designs, General block designs in finite geometry
orthogonal double cover, symmetric balanced incomplete block design, symmetric graph design, Combinatorial aspects of block designs, General block designs in finite geometry
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