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Self-complementary hypergraphs

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A k-uniform hypergraph H = (V;E) is called self-complementary if there is a permutation σ:V → V, called self-complementing, such that for every k-subset e of V, e ∈ E if and only if σ(e) ∉ E. In other words, H is isomorphic with $H' = (V; \binom{V}{k} - E)$. In the present paper, for every k, (1 ≤ k ≤ n), we give a characterization of self-complementig permutations of k-uniform self-complementary hypergraphs of the order n. This characterization implies the well known results for self-complementing permutations of graphs, given independently in the years 1962-1963 by Sachs and Ringel, and those obtained for 3-uniform hypergraphs by Kocay, for 4-uniform hypergraphs by Szymański, and for general (not uniform) hypergraphs by Zwonek.
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17th workshop '3in1' 2008

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Bipartite graphs G = (L,R;E) and H = (L',R';E') are bi-placeabe if there is a bijection f:L∪R→ L'∪R' such that f(L) = L' and f(u)f(v) ∉ E' for every edge uv ∈ E. We prove that if G and H are two bipartite balanced graphs of order |G| = |H| = 2p ≥ 4 such that the sizes of G and H satisfy ||G|| ≤ 2p-3 and ||H|| ≤ 2p-2, and the maximum degree of H is at most 2, then G and H are bi-placeable, unless G and H is one of easily recognizable couples of graphs. This result implies easily that for integers p and k₁,k₂,...,kₗ such that $k_i ≥ 2$ for i = 1,...,l and k₁ +...+ kₗ ≤ p-1 every bipartite balanced graph G of order 2p and size at least p²-2p+3 contains mutually vertex disjoint cycles $C_{2k₁},...,C_{2kₗ}$, unless $G = K_{3,3} - 3K_{1,1}$.
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We prove that a k-uniform self-complementary hypergraph of order n exists, if and only if $\binom{n}{k}$ is even.
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Pₘ-saturated bipartite graphs with minimum size

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A graph G is said to be H-saturated if G is H-free i.e., (G has no subgraph isomorphic to H) and adding any new edge to G creates a copy of H in G. In 1986 L. Kászonyi and Zs. Tuza considered the following problem: for given m and n find the minimum size sat(n;Pₘ) of Pₘ-saturated graph of order n. They gave the number sat(n;Pₘ) for n big enough. We deal with similar problem for bipartite graphs.
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