Monday, 3 July 2006

transcendence - Showing e is transcendental using its continued fraction expansion

In his article Transcendental continued fractions, Journal of Number Theory 13, November 1981, 456-462, Gideon Nettler shows that two numbers given by continued fractions
$A = [a_0,a_1,a_2,...]$ and $B = [b_0, b_1, b_2, ...]$ have the property that $A$, $B$,
$A pm B$, $A/B$ and $AB$ are irrational if $frac12 a_n > b_n > a_{n-1}^{5n}$ for sufficiently large $n$, and transcendental if $a_n > b_n > a_{n−1}^{(n−1)^2}$ for sufficiently large $n$. The growth of the $a_i$ in the continued fraction expansion of $e$ is so small that present methods seem useless for proving the transcendence of $e$ in this way.



Edit. Similarly, Alan Baker proved in Continued fractions of transcendental numbers (Mathematika 9 (1962), 1-8) that if $q_n$ denotes the denominator of the $n$-th convergent of a continued fraction $A$, and if
$$ lim sup frac{(log log q_n)(log n)^{1/2}}{n} = infty, $$
then $A$ is transcendental.



Edit 2 I guess that the answer to your question should be a firm "yes" after all.
In



  • Über einige Anwendungen diophantischer Approximationen,
    Abh. Preuss. Akad. Wiss. 1929; Gesammelte Abhandlungen, vol I, p. 209-241

Siegel proved that all continued fractions
$$ frac{1}{a_1 + cfrac{1}{a_2 + cfrac{1}{a_3 + ldots}}} $$
in which the $a_i$ form a nonconstant arithmetic sequence are transcendental.
Applying this to the continued fraction expansion of $frac{e-1}{e+1}$ gives
the transcendence of $e$.



Siegel's proof uses, predictably, analytic machinery (solutions of Bessel and
Riccati differential equations) going far beyond Liouville's theorem.

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