Liouville number

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In number theory, a Liouville number is a real number with the property that, for every positive integer , there exists a pair of integers with such that

The inequality implies that Liouville numbers possess an excellent sequence of rational number approximations. In 1844, Joseph Liouville proved a bound showing that there is a limit to how well algebraic numbers can be approximated by rational numbers, and he defined Liouville numbers specifically so that they would have rational approximations better than the ones allowed by this bound. Liouville also exhibited examples of Liouville numbers[1] thereby establishing the existence of transcendental numbers for the first time.[2] One of these examples is Liouville's constant

in which the nth digit after the decimal point is 1 if is the factorial of a positive integer and 0 otherwise. It is known that π and e, although transcendental, are not Liouville numbers.[3]

The existence of Liouville numbers (Liouville's constant)

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Liouville numbers can be shown to exist by an explicit construction.

For any integer   and any sequence of integers   such that   for all   and   for infinitely many  , define the number

 

In the special case when  , and   for all  , the resulting number   is called Liouville's constant:

 

It follows from the definition of   that its base-  representation is

 

where the  th term is in the  th place.

Since this base-  representation is non-repeating it follows that   is not a rational number. Therefore, for any rational number  ,  .

Now, for any integer  ,   and   can be defined as follows:

 

Then,

 

Therefore, any such   is a Liouville number.

Notes on the proof

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  1. The inequality   follows since ak ∈ {0, 1, 2, ..., b−1} for all k, so at most ak = b−1. The largest possible sum would occur if the sequence of integers (a1, a2, ...) were (b−1, b−1, ...), i.e. ak = b−1, for all k.   will thus be less than or equal to this largest possible sum.
  2. The strong inequality   follows from the motivation to eliminate the series by way of reducing it to a series for which a formula is known. In the proof so far, the purpose for introducing the inequality in #1 comes from intuition that   (the geometric series formula); therefore, if an inequality can be found from   that introduces a series with (b−1) in the numerator, and if the denominator term can be further reduced from  to  , as well as shifting the series indices from 0 to  , then both series and (b−1) terms will be eliminated, getting closer to a fraction of the form  , which is the end-goal of the proof. This motivation is increased here by selecting now from the sum   a partial sum. Observe that, for any term in  , since b ≥ 2, then  , for all k (except for when n=1). Therefore,   (since, even if n=1, all subsequent terms are smaller). In order to manipulate the indices so that k starts at 0, partial sum will be selected from within   (also less than the total value since it's a partial sum from a series whose terms are all positive). Choose the partial sum formed by starting at k = (n+1)! which follows from the motivation to write a new series with k=0, namely by noticing that  .
  3. For the final inequality  , this particular inequality has been chosen (true because b ≥ 2, where equality follows if and only if n=1) because of the wish to manipulate   into something of the form  . This particular inequality allows the elimination of (n+1)! and the numerator, using the property that (n+1)! – n! = (n!)n, thus putting the denominator in ideal form for the substitution  .

Irrationality

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Here the proof will show that the number   where c and d are integers and   cannot satisfy the inequalities that define a Liouville number. Since every rational number can be represented as such  the proof will show that no Liouville number can be rational.

More specifically, this proof shows that for any positive integer n large enough that   [equivalently, for any positive integer  )], no pair of integers   exists that simultaneously satisfies the pair of bracketing inequalities

 

If the claim is true, then the desired conclusion follows.

Let p and q be any integers with   Then,

 

If   then

 

meaning that such pair of integers   would violate the first inequality in the definition of a Liouville number, irrespective of any choice of n .

If, on the other hand, since   then, since   is an integer, we can assert the sharper inequality   From this it follows that

 

Now for any integer   the last inequality above implies

 

Therefore, in the case   such pair of integers   would violate the second inequality in the definition of a Liouville number, for some positive integer n.

Therefore, to conclude, there is no pair of integers   with   that would qualify such an   as a Liouville number.

Hence a Liouville number cannot be rational.

Liouville numbers and transcendence

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No Liouville number is algebraic. The proof of this assertion proceeds by first establishing a property of irrational algebraic numbers. This property essentially says that irrational algebraic numbers cannot be well approximated by rational numbers, where the condition for "well approximated" becomes more stringent for larger denominators. A Liouville number is irrational but does not have this property, so it can't be algebraic and must be transcendental. The following lemma is usually known as Liouville's theorem (on diophantine approximation), there being several results known as Liouville's theorem.

Lemma: If   is an irrational root of an irreducible polynomial of degree   with integer coefficients, then there exists a real number   such that for all integers   with  ,

 

Proof of Lemma: Let   be a minimal polynomial with integer coefficients, such that  .

By the fundamental theorem of algebra,   has at most   distinct roots.
Therefore, there exists   such that for all   we get  .

Since   is a minimal polynomial of   we get  , and also   is continuous.
Therefore, by the extreme value theorem there exists   and   such that for all   we get  .

Both conditions are satisfied for  .

Now let   be a rational number. Without loss of generality we may assume that  . By the mean value theorem, there exists   such that

 

Since   and  , both sides of that equality are nonzero. In particular   and we can rearrange:

 

Proof of assertion: As a consequence of this lemma, let x be a Liouville number; as noted in the article text, x is then irrational. If x is algebraic, then by the lemma, there exists some integer n and some positive real A such that for all p, q

 

Let r be a positive integer such that 1/(2r) ≤ A and define m = r + n. Since x is a Liouville number, there exist integers a, b with b > 1 such that

 

which contradicts the lemma. Hence a Liouville number cannot be algebraic, and therefore must be transcendental.

Establishing that a given number is a Liouville number proves that it is transcendental. However, not every transcendental number is a Liouville number. The terms in the continued fraction expansion of every Liouville number are unbounded; using a counting argument, one can then show that there must be uncountably many transcendental numbers which are not Liouville. Using the explicit continued fraction expansion of e, one can show that e is an example of a transcendental number that is not Liouville. Mahler proved in 1953 that π is another such example.[4]

Uncountability

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Consider the number

3.1400010000000000000000050000....

3.14(3 zeros)1(17 zeros)5(95 zeros)9(599 zeros)2(4319 zeros)6...

where the digits are zero except in positions n! where the digit equals the nth digit following the decimal point in the decimal expansion of π.

As shown in the section on the existence of Liouville numbers, this number, as well as any other non-terminating decimal with its non-zero digits similarly situated, satisfies the definition of a Liouville number. Since the set of all sequences of non-null digits has the cardinality of the continuum, the same is true of the set of all Liouville numbers.

Moreover, the Liouville numbers form a dense subset of the set of real numbers.

Liouville numbers and measure

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From the point of view of measure theory, the set of all Liouville numbers   is small. More precisely, its Lebesgue measure,  , is zero. The proof given follows some ideas by John C. Oxtoby.[5]: 8 

For positive integers   and   set:

 

then

 

Observe that for each positive integer   and  , then

 

Since

 

and   then

 

Now

 

and it follows that for each positive integer  ,   has Lebesgue measure zero. Consequently, so has  .

In contrast, the Lebesgue measure of the set of all real transcendental numbers is infinite (since the set of algebraic numbers is a null set).

One could show even more - the set of Liouville numbers has Hausdorff dimension 0 (a property strictly stronger than having Lebesgue measure 0).

Structure of the set of Liouville numbers

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For each positive integer n, set

 

The set of all Liouville numbers can thus be written as

 

Each   is an open set; as its closure contains all rationals (the   from each punctured interval), it is also a dense subset of real line. Since it is the intersection of countably many such open dense sets, L is comeagre, that is to say, it is a dense Gδ set.

Irrationality measure

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The Liouville–Roth irrationality measure (irrationality exponent, approximation exponent, or Liouville–Roth constant) of a real number   is a measure of how "closely" it can be approximated by rationals. It is defined by adapting the definition of Liouville numbers: instead of requiring the existence of a sequence of pairs   that make the inequality hold for each  —a sequence which necessarily contains infinitely many distinct pairs—the irrationality exponent   is defined to be the supremum of the set of   for which such an infinite sequence exists, that is, the set of   such that   is satisfied by an infinite number of integer pairs   with  .[6]: 246  For any value  , the infinite set of all rationals   satisfying the above inequality yields good approximations of  . Conversely, if  , then there are at most finitely many   with   that satisfy the inequality. If   is a Liouville number then  .

See also

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References

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  1. ^ Joseph Liouville (May 1844). "Mémoires et communications". Comptes rendus de l'Académie des Sciences (in French). 18 (20, 21): 883–885, 910–911.
  2. ^ Baker, Alan (1990). Transcendental Number Theory (paperback ed.). Cambridge University Press. p. 1. ISBN 978-0-521-39791-9.
  3. ^ Baker 1990, p. 86.
  4. ^ Kurt Mahler, "On the approximation of π", Nederl. Akad. Wetensch. Proc. Ser. A., t. 56 (1953), p. 342–366.
  5. ^ Oxtoby, John C. (1980). Measure and Category. Graduate Texts in Mathematics. Vol. 2 (Second ed.). New York-Berlin: Springer-Verlag. doi:10.1007/978-1-4684-9339-9. ISBN 0-387-90508-1. MR 0584443.
  6. ^ Bugeaud, Yann (2012). Distribution modulo one and Diophantine approximation. Cambridge Tracts in Mathematics. Vol. 193. Cambridge: Cambridge University Press. doi:10.1017/CBO9781139017732. ISBN 978-0-521-11169-0. MR 2953186. Zbl 1260.11001.
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