Tuesday, August 6, 2013

are there more irrational numbers than rational numbers? or are they the same?

The answer here is that there are in fact far, far more
irrational numbers than there are rational numbers. One way to think about this is that
between any two rational numbers, there are an infinite number of irrational
numbers.


There is quite a bit of background knowledge
required to understand the answer to this question, and I will attempt to give an
overview. We must first define a few terms. We call the number of elements in a set
the cardinality of the set. For example, the set {1, 2, 3} has
cardinality 3. Two sets are said to have the same cardinality if a bijection can be
formed between the sets. We can extend the notion of cardinality to infinite sets, and
we say that the set of natural numbers `NN` has cardinality `aleph_0` (aleph
null).


Any set that has cardinality `aleph_0` is said to
be countably infinite. The set of all real numbers `RR` was
famously shown by Georg Cantor to have cardinality `aleph_1 = 2^(aleph_0)` . (See
reference link "Cantor's diagonal argument"). This number is called
the cardinality of the continuum, and a set with this cardinality
is said to be uncountably infinite.


It
has been shown that the irrational numbers are uncountably infinite
(they have cardinality `aleph_1` ).


However, the
rational numbers are countably infinite (have cardinality
`aleph_0`). To show that a set has cardinality `aleph_0`, you must construct a bijection
(one-to-one and onto function) between the set in question and the natural numbers.
An bijection between the rational numbers and the natural numbers is shown in reference
[3]. As such, we know that the rational numbers have cardinality
`aleph_0`.


It follows that there are more
irrational numbers than rational numbers, since there are `aleph_0` rational numbers and
`aleph_1` irrational numbers, and `aleph_0 < aleph_1 = 2^(aleph_0)`
.

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