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.
Keywords cluster map (TF-IDF): A shape represents a cluster, color means the time of the first occurrence, the color gradually changes from cool to warm, blue indicates the earlier year, red indicates the warmest color for the most recent year. The smaller the cluster tag number, the higher the number of published literature and the frequency of citation.
