302. Some problems with sequencies of the "roots" .

by Virgil Nicula, Jul 24, 2011, 2:40 AM

Some problems with sequencies of the "roots" . See here an example (the "problem of yesterday").

Show that for every $n\in N,\ n\ge 3$ there is exactly one $x_n\ge 0$ so that $f(x_n)=0$, where:

$1.\ f_1(x)=x^n-x-n\ : \ \ x_n\rightarrow 1,\ \frac{\ln n}{n(x_n-1)}\rightarrow 1\ ;$

$2.\ f_2(x)=x^n-nx+1-n\ : \ \ x_n\rightarrow 1\ ;$

$3.\ f_3(x)=x^{n+2}-(n+2)x-(n+1)\ : \ \ x_n\rightarrow 1\ ;$

$4.\ f_4(x)=x^n+nx-1\ : \ \ x_n\rightarrow 0,\ \ (1+x_n)^n\rightarrow e\ ;$

$5.\ f_5(x)=x^n+px-1\ (p\in N^*)\ : \ x_n\rightarrow \frac 1p\ ;$

$6.\ f_6(x)=x^n-\sum\limits_{k=0}^{n-1} x^k\ : \ x_n\rightarrow 2\ ;$

$7.\ f_7(x)=x^n+x^{n-1}+\ldots +x^3+x^2+2x-1\ : \ x_n\rightarrow \frac{3-\sqrt 5}{2}\ ;$

$8.\ f_8(x)=x^n+\arctan x -1\ : \ x_n\rightarrow 1\ ;$

$9.\ f_9(x)=x^n+\ln x\ : \ x_n\rightarrow 1\ ;$

$10.\ f_{10}(x)=e^{-x}+nx^2-2x-1: \ nx_n\rightarrow 3\ ;$

$11.\ f_{11}(x)=\left( 1+\frac 1n\right)^{n+x} -\sum\limits_{k=0}^n \frac{1}{k!}\ : \ x_n\rightarrow \frac 12\ ;$

$12.\ f_{12}(x)=\ln (n+x)+E-\sum\limits_{k=1}^n \frac 1k$ (E - Euler's constant) $\ : \ x_n\rightarrow \frac 12$ .


Proof.

See
here and here
This post has been edited 6 times. Last edited by Virgil Nicula, Nov 21, 2015, 7:19 AM

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