
 Number  Wikipedia, the free encyclopedia 
  The first existence proofs of irrational numbers is usually attributed to Pythagoras, more specifically to the Pythagorean Hippasus of Metapontum, who produced a (most likely geometrical) proof of the irrationality of the square root of 2. 
  This number is denoted by i, a symbol assigned by Leonhard Euler. 
  The arithmetical operations of numbers, such as addition, subtraction, multiplication and division, are generalized in the branch of mathematics called abstract algebra, the study of abstract number systems such as groups, rings and fields. 
 en.wikipedia.org /wiki/Number (3655 words) 
