A mobile number is an element of a directed permutation with a smaller neighboring element in its assigned direction. Zero is never mobile. section 6.2.5
Even’s change-ringing algorithm uses mobile numbers to generate permutations:
- Find the largest mobile number. section 6.2.5
- Swap it with the adjacent element in its direction. section 6.2.5
- Reverse the directions of every number larger than the moved number. section 6.2.5
- Stop when no mobile number remains. section 6.2.5
Reading mobility
In ←0 ←1 ←2 ←3, numbers 1, 2, and 3 are mobile. In ←1 2→ ←3 ←0, only 3 is mobile because it alone has a smaller neighbor in its direction. section 6.2.5
