There are several hashing-based data structures whose space utilization (keys per table cells) directly depends on the edge density threshold for the appearance of a 2-core in some underlying random k-uniform hypergraph. We show that by modifying these data structures such that the k-uniform hypergraphs are replaced by certain non-uniform hypergraphs their space utilization can be improved. These non-uniform hypergraphs are a mixture of uniform hypergraphs each with a linear number of edges but with different edge sizes. In the case of two different edge sizes we give a solution for the optimal (expected) number of edges of each size such that the 2-core threshold for the resulting mixed hypergraph is maximized. For suitable edge sizes we obtain optimal thresholds for mixed hypergraphs up to 0.920, improving the maximum 2-core threshold for any random k-uniform hypergraph, which is about 0.818.
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