Can the boundaries of two pentagons intersect at $20$ points?
Clash Royale CLAN TAG #URR8PPP up vote 16 down vote favorite 8 This question is a follow-up to Maximum number of intersections between a quadrilateral and a pentagon, where it is shown that the boundaries $partial Q,partial P$ of a quadrilateral and a pentagon in the plane cannot intersect at more than $16$ points, since each side of $partial Q$ meets $partial P$ at an even number of points. Q: Given the boundaries $partial P_1, partial P_2$ of two pentagons in the plane, is it possible that $$ left|partial P_1 cap partial P_2 right| = 20?$$ Each side of $partial P_1$ meets $partial P_2$ at an even number of points, so equality is attained iff there is some configuration such that each side of $partial P_1$ meets each side of $partial P_2$ except one. $left|partial P_1 cap partial P_2right| = 18$ is possible, as shown below, and I believe that $left|partial P_1 cap partial P_2right| = 20$ is im possible, but I am failing to prove it. combinatorics curves intersectio...