Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Schematic depiction of unwrapping states where site k (yellow) is (a) inaccessible and (b) accessible. The number of sites opened from the left and right are given by i and j, respectively. Δ is the number of additional sites that need to be opened beyond a site to make it accessible. Same colour code as in fig. 1.
