Fig 3. Number of papers per year for each of the research fronts.
A: Research fronts whose number of papers peaked between 1990 and 1991, B: peaked between 1996 and 1999 and C: peaked between 2004 and 2007.
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.