Unlike the derogatory implication of the word ‘discrimination’ in human affairs, for examples: job discrimination, age discrimination, sex discrimination, race discrimination, intelligence discrimination, social discrimination, class discrimination, etc. on the contrary number discrimination was and still is crucial for the progress and successful evolution of the science of mathematics.

One math popularizers by the name of John Derbyshire thought of a memorable mnemonic: Nine zulu queens ruled china. The hollow letter N denotes the set of natural numbers which is the smallest set of all the sets of discriminated numbers. Analogously, N represents the innermost nested Russian dolls for numbers. The next encompassing doll is represented by the hollow letter Z denoting the set of integers including zero and negative whole numbers. The next encompassing Russian doll is represented by the hollow letter Q which denotes the set of rational numbers. The next doll represented by the hollow letter R represents the entire set of real numbers which include the set of irrational numbers. The outermost Russian doll represented by the hollow letter C represents the set of complex numbers including the set of all imaginary numbers. These categorical levels for number discrimination evolved gradually within a time span no more than three thousand years. John Derbyshire’s Bogus History of Numbers can be found on page 174 of his book Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics. http://www.olimu.com/