Using other words we can say that it will be licit to construct a subset b of a from a property A (x). We write: b = {x Î a: A (x)}, and read "b is the subset of a formed by the sets x that belong to a and satisfy the property A". When the Singer Set Theory, called the Intuitive Set Theory, came up, there was the idea that “any property” could be considered so that the objects that satisfied it formed a set.

The number Pi represents the value of the ratio between the circumference of any circle and its diameter. It is the oldest known mathematical constant. It is an irrational number, with infinite decimal places and not periodic. What are cyclic numbers? Index Next >> What are Mersenne Numbers?

Row of Bottles Difficulty Level: There are nine bottles in a row. The first five are full of beer and the last four empty. Moving only two bottles, like making the row with alternately full and empty bottles. Challenge 41 Solve the equation Challenge Index Next >> Challenge 43 How Many Eggs a Year?