Figure 4.
Co-citation network of sources.
Red cluster = cluster 1; blue cluster = cluster 2; green cluster = cluster 3.
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.