The Complexity of Simultaneous Geometric Graph Embedding
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Given a collection of planar graphs G_1,...,G_k on the same set V of n vertices, the simultaneous geometric embedding (with mapping) problem, or simply k-SGE, is to find a set P of n points in the plane and a bijection ϕ: V → P such that the induced straight-line drawings of G_1,...,G_k under ϕ are all plane. This problem is polynomial-time equivalent to weak rectilinear realizability of abstract topological graphs, which Kynčl (doi:10.1007/s00454-010-9320-x) proved to be complete for ∃R, the existential theory of the reals. Hence the problem k-SGE is polynomial-time equivalent to several other problems in computational geometry, such as recognizing intersection graphs of line segments or finding the rectilinear crossing number of a graph. We give an elementary reduction from the pseudoline stretchability problem to k-SGE, with the property that both numbers k and n are linear in the number of pseudolines. This implies not only the ∃R-hardness result, but also a 2^2^Ω (n) lower bound on the minimum size of a grid on which any such simultaneous embedding can be drawn. This bound is tight. Hence there exists such collections of graphs that can be simultaneously embedded, but every simultaneous drawing requires an exponential number of bits per coordinates. The best value that can be extracted from Kynčl's proof is only 2^2^Ω (√(n)).
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