現代数学の系譜工学物理雑談古典ガロア理論も読む46

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203: １３２人目の素数さん [sage] 2017/11/16(木) 13:53:40.78 ID:/MLxWF5k

>>200 補足
http://www.unirioja.es/cu/jvarona/downloads/Differentiability-DA-Roth.pdf
DIFFERENTIABILITY OF A PATHOLOGICAL FUNCTION,
DIOPHANTINE APPROXIMATION,
AND A REFORMULATION
OF THE THUE-SIEGEL-ROTH THEOREM 2009
抜粋引用
In the opinion of this author, fν is a very
interesting function, and it is worthwhile to continue analyzing its behaviour.
In this way, we find examples of functions whose properties about con-
tinuity and dierentiability are pathological at the same time. For every
ν > 0, the function fν is continuous at the irrationals and discontinuous
at the rationals. And, when ν > 2 (that is the most interesting case), we
prove that fν is dierentiable in a set Dν

It is astonishing that, dierentiability being a local concept, fν is dieren-
tiable almost everywhere in spite of the fact that it is not continuous at any
rational number.
We finish the paper by showing a reformulation of the Thue-Siegel-Roth
theorem in terms of the dierentiability of fν for ν > 2 (see Theorem 3
and the final Remark). It seems really surprising that a theorem about dio-
phantine approximation is equivalent to another theorem about the dier-
entiablity of a real function: a nice new connection between number theory
and analysis! As far as I know, this characterization of the Thue-Siegel-Roth
theorem has not been previously observed.

Remark 1. The pathological behavior of functions is a useful source of
examples that help to understand the rigorous definitions of the basic con-
cepts in mathematical analysis. In this respect, it is interesting to note that,
here, we have shown a kind of pathological behaviour that is dierent from
that of the more commonly studied: the existence of continuous nowhere
dierentiable real functions, whose most typical example is the Weierstrass
function

4. The theorem of Thue-Siegel-Roth revisited

http://rio2016.5ch.net/test/read.cgi/math/1510442940/203

596: 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE  [sage] 2017/11/26(日) 23:26:10.78 ID:1WQ1V5QH

>>575 補足

原本PDFを見て貰った方が視認性は良いが、後の検索性のためにコピペする（＾＾
http://www.unirioja.es/cu/jvarona/downloads/Differentiability-DA-Roth.pdf
DIFFERENTIABILITY OF A PATHOLOGICAL FUNCTION, DIOPHANTINE APPROXIMATION, AND A REFORMULATION OF THE THUE-SIEGEL-ROTH THEOREM JUAN LUIS VARONA 2009
(抜粋)
P7
4. The theorem of Thue-Siegel-Roth revisited

Or, equivalently, if x is an irrational algebraic number, there exists a positive constant C(x, α) such
that ｜x - p/q ｜< C(x, α)/q^(2+α) (10)
has no rational solution.

P8
Remark 3. We have proved Theorem 3 by using the Thue-Siegel-Roth theorem.
But we have said that it is a reformulation. So, let us see how to
deduce the Thue-Siegel-Roth theorem from Theorem 3.
Given x algebraic and irrational, and ν > 2, Theorem 3 ensures that fν
is differentiable at x, so there exists
lim y→x {fν(y) - fν(x)}/(y - x) = f’ν (x).
By approximating y → x by irrationals y, it follows that f’ν (x) = 0.
Consequently, by approximating y → x by rationals, i.e., y = p/q, we also must have
lim p/q→x ｛fν(p/q) - fν(x)｝/(p/q - x ) = lim p/q→x (1/qν)/(p/q - x) = 0.
Then, for every ε > 0, there exists δ > 0 such that
1/(q^ν) <=  ε｜p/q - x｜
when p/q ∈ (x - δ, x + δ). From here, it is easy to check that the same
happens for every p/q ∈ Q, perhaps with a greather constant ε' in the place
of ε. Thus, (10) with α = ν-2 and some positive constant C(x, α) = 1/ε' has
no rational solution, and we have obtained the Thue-Siegel-Roth theorem.
(引用終り)

つづく

http://rio2016.5ch.net/test/read.cgi/math/1510442940/596

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