The delunification process and minimal diagrams

Abstract

A link diagram is said to be lune-free if, when viewed as a 4-regular plane graph it does not have multiple edges between any pair of nodes. We prove that any colored link diagram is equivalent to a colored lune-free diagram with the same number of colors. Thus any colored link diagram with a minimum number of colors (known as a minimal diagram) is equivalent to a colored lune-free diagram with that same number of colors. We call the passage from a link diagram to an equivalent lune-free diagram its delunification process.We then introduce a notion of Grey Sets in order to obtain lower bounds for minimum number of colors. We calculate these lower bounds for a number of prime moduli with the help of computer programs.For each number of crossings through 16, we list the lune-free diagrams and we color them. If the number of colors equals the corresponding lower bound we know we have a minimum number of colors. We also introduce and list the lune-free crossing number of a link i.e., the minimum number of crossings needed for a lune-free diagram of this link, and other related link invariants.