6 1. TROPICS (0, 0) x1 0 2 (1, −1) 1 + x2 2x1 + 2x2 Figure 5. 0 (0 x1) (1 x2) (0 x1 x1 x2 x2) (0, 1) ˜ S (3, 2) (1, 0) (2, 0) 0 1 2 3 ΔS Figure 6 the convex hull of S in NR. The coefficients an then define a function ϕ : ΔS R as follows. We consider the upper convex hull ˜ S of the set ˜ = {(n, an) | n S} NR × R, namely ˜ S = {(n, a) NR × R | there exists (n, a ) Conv( ˜) with a a }. We then define ϕ(n) = min{a R | (n, a) ˜ S }. For example, considering the univariate tropical polynomial f = 1 (0 x) (0 x2) (2 x3), we get ΔS and ˜ S as depicted in Figure 6, with the lower boundary of ˜ S being the graph of ϕ. This picture yields a polyhedral decomposition of ΔS: Definition 1.3. A (lattice) polyhedral decomposition of a (lattice) polyhedron Δ NR is a set P of (lattice) polyhedra in NR called cells such that
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