Arrangements of Approaching Pseudo-Lines.

We consider arrangements of n pseudo-lines in the Euclidean plane where each pseudo-line ℓ i is represented by a bi-infinite connected x -monotone curve f i ( x ) , x ∈ R , such that for any two pseudo-lines ℓ i and ℓ j with i < j , the function x ↦ f j ( x ) - f i ( x ) is monotonically decreasing and surjective (i.e., the pseudo-lines approach each other until they cross, and then move away from each other). We show that such arrangements of approaching pseudo-lines , under some aspects, behave similar to arrangements of lines, while for other aspects, they share the freedom of general pseudo-line arrangements. For the former, we prove:There are arrangements of pseudo-lines that are not realizable with approaching pseudo-lines.Every arrangement of approaching pseudo-lines has a dual generalized configuration of points with an underlying arrangement of approaching pseudo-lines. For the latter, we show:There are 2 Θ ( n 2 ) isomorphism classes of arrangements of approaching pseudo-lines (while there are only 2 Θ ( n log n ) isomorphism classes of line arrangements).It can be decided in polynomial time whether an allowable sequence is realizable by an arrangement of approaching pseudo-lines. Furthermore, arrangements of approaching pseudo-lines can be transformed into each other by flipping triangular cells, i.e., they have a connected flip graph, and every bichromatic arrangement of this type contains a bichromatic triangular cell.