IEEE Computational Intelligence Magazine - February 2020 - 94

positions of the research areas are determined by the co-occurrence frequency
of the keywords.
IV. Country Wise Distribution

Many countries have researchers working
in the area of T2 FSs. Here we study various aspects of the contributions by
researchers from different regions/countries. It includes country wise distribution, time dynamics, impact analysis,
institution analysis, etc.
A. Country Level-Analysis

At present, researchers from 68 countries have published on T2 FSs. More
than 50% of the publications have been
contributed by the researchers of only
four countries including: China, Iran,
USA, and Taiwan. Out of these, Chinese researchers have produced the
largest number of papers (TP = 274). It
is approximately a fifth of the total
share of all papers in T2 FS (21.2%).
Iranian researchers stand at the second
position with the share of 13.04%, hav-

ing published 168 papers. The other
productive countries with researchers
on T2 FSs, after China and Iran, are
USA, Taiwan, United Kingdom, and
Turkey with not much differentiated
shares of 12.8%, 10.7%, 8.77%, and
8.22% having 165, 138, 113 and 106
publications, respectively.
B. Impact Analysis

In this section, we analyze the quality of
the publications (measured in terms of
citations) by researchers from different
countries, divided into four periods (1997-
2001), (2002-2006), (2007-2011), (2012-
2017).The countries are sorted on the basis
of the metric TC. The top six countries in
terms of citations are USA, UK, China,
Taiwan, Mexico and Spain, in order.
If we look at the most recent period,
countries with the highest citations are
China, Mexico, Taiwan, UK, USA and
Spain. Fig. 3 shows the country-level
citations in the 4 periods (1997-2001),
(2002-2006), (2007-2011), (2012-2017)
for research on T2 FSs.

FIGURE 2 Topic density visualization using VOSviewer.

IEEE COMPUTATIONAL INTELLIGENCE MAGAZINE | FEBRUARY 2020

C. Growth Rate

T2 FSs originated in the USA and
researchers in the country published the
largest number of papers in the initial 15
years. The number of papers from the
researchers in USA has declined in
recent years while researchers in other
countries like China and Taiwan have
published more. In Fig. 4, we see the
total production plotted against the
average annual productivity growth in
T2 FSs for researchers in different countries. Researchers in China have published the largest number of papers
(>200) and also has a fairly high growth
rate of 15.5%. Researchers in Iran have
shown a considerably higher growth
rate of 13.4% in the last decade.
However, researchers in USA and
Taiwan have more than a hundred
papers each but the growth rate has
been low (5% for Taiwanese researchers
and -22.3% for researchers in the USA).
Publications by the researchers in UK
also have a low growth rate of -8.3%.
Works published by the researchers in

IEEE Computational Intelligence Magazine - February 2020

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