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.
Author co-occurrence cluster map by CiteSpace. Each node represents one author. Lines indicate co-occurrence relationships. The color corresponds to the time. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
