I distinctly remember one day in second grade when the day after our first class on multiplication my friend infamously claimed that

Although he was definitely incorrect in his thinking and reasoning, because division be zero has no meaning when it comes to real numbers (or integers), there are cases, such asĀ The Riemann Sphere or a one-element field where the multiplicative identity coincides with the additive identity, when the equation above is actually true (and well defined).

It is sometimes quite tempting (and even useful) to consider

The main problem with allowing the division by zero is that it results in logical fallacies such as 1=0 (actually, one can equate any two numbers using it). Eventually, mathematicians found a way to define limits rigorously without infinitesimal quantities needed by Newton and Leibnitz when Cauchy and Weierstrass laid down the foundation of modern analysis.

The next question from Yahoo Answers is related to a different kind of infinity – the kind that deals with sets. The works of Georg Cantor between 1874 and 1884 are the origin of set theory in which he established the importance of one-to-one correspondence between sets, defined infinite and well-ordered sets, and proved that the real numbers are “more infinite” than the natural numbers. The diagonalization argument he used demonstrates a powerful and general technique that has since been used in a wide range of proofs such as this one.

**Problem**: Determine whether the set of real numbers between 0 and 1 with decimal representations consisting of 1s, i.e. is countable or uncountable?

Definitions:

- A function is
*bijective*if and only if for every there is*exactly*one such that - A set is said to be
*countable*if there exists a*bijective*function where is the set of natural numbers.

**Solution**:

Let us look at set and consider the following list:

It is easy to see that our set can be easily put into a *one-to-one* correspondence with the set of natural numbers , e.g. the correspondence is given by the list (function defined) above.

To be precise, let be given by

To show that is bijective, we need to show thatĀ for every there is exactly one, i.e. a unique, such that .

Consider an arbitrary . Since

we have

and so by taking we get

Therefore, for every we have proven the existance of (at least one) such that .

To show uniqueness, we need to demonstrate that if then . The easiest way to accomplish this is to use proof by *contrapositive*, that is, the fact that

is equivalent to

Let be such that . Without loss of generality, assume (otherwise rename them). Then it follows that there exist such that . Therefore

Since it follows that and so we have

thus proving the uniqueness. Therefore is a bijective function and so is countable.