What category does a collection of mathematics tricks come under?
A number of people have correctly told you (I can see the article) that this is a collection of mathematical puzzles, rather than a research paper.  So let me tell you a little more about this type of article.  Martin Gardner is considered by many to be the biggest popularizer of recreational mathematics in the 20th century (perhaps ever).  For 25 years (1956-1981), he wrote a column for Scientific American called Mathematical Games.  Many of his columns have been collected into books (such as Martin Gardner's Mathematical Games).  Some other well-known examples of similar writing include that of Douglas Hofstadter, Ian Stewart, and Keith Devlin.