Processing math: 100%
Dual cir­cum­fer­ence and free sets
Pat Morin
Car­leton Uni­ver­sity

For a pla­nar graph G, a free set is a sub­set F\subset V(G) such that, for any set X of |F| points in \mathbb{R}^2, there is a non-cross­ing straight-line draw­ing of G in which the points in F are drawn on the points of X. The ex­is­tence of large free sets in n-ver­tex pla­nar graphs has been used to give the tight­est re­sults for a num­ber of prob­lems in graph draw­ing and geo­met­ric graph the­ory in­clud­ing un­tan­gling, col­umn pla­narity, uni­ver­sal point sub­sets, and par­tial si­mul­ta­ne­ous geo­met­ric draw­ings. For the last ten years, progress on these prob­lems has been lim­ited by the fact that the best known lower bound on the size of free sets in n-ver­tex pla­nar graphs is \Omega(n^{0.5}).

In this talk, I will show that if G is a tri­an­gu­la­tion of max­i­mum de­gree \Delta and the dual of G con­tains a sim­ple cycle of length \ell, then G has a free set F of size \Omega(\ell/\Delta^4). This, along with the cur­rent best lower-bounds on the cir­cum­fer­ence of 3-reg­u­lar 3-con­nected pla­nar graphs shows that every n-ver­tex pla­nar graph of max­i­mum de­gree \Delta has a free set of size \Omega(n^{0.8}/\Delta^4). This gives im­proved re­sults for all the ap­pli­ca­tions men­tioned above in the class of pla­nar graphs of bounded de­gree.