17/11/16 13:53:40.78 /MLxWF5k.net
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URLﾘﾝｸ(www.unirioja.es)
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