# Help on Hint

• Feb 21st 2010, 02:45 PM
mabruka
Help on Hint
hi I need some help understanding the hint this exercise gives:

http://img695.imageshack.us/img695/6521/problemaa.jpg

Where$\displaystyle I=[-2,2]$ and
$\displaystyle A_1=\{x \in I : Q_c(x)\in I \}$.

$\displaystyle p_+$ is the fixed point of $\displaystyle Q_c$ in $\displaystyle [0,2]$.

$\displaystyle p_+=\frac{1+ \sqrt{1-4c}}{2}$

I dont understand why it suffices to find the c-values the hint points to guarantee the conclusion for $\displaystyle x \in [0,\frac{1}{2}]$

thank you!
• Feb 21st 2010, 06:38 PM
tonio
Quote:

Originally Posted by mabruka
hi I need some help understanding the hint this exercise gives:

http://img695.imageshack.us/img695/6521/problemaa.jpg

Where$\displaystyle I=[-2,2]$ and
$\displaystyle A_1=\{x \in I : Q_c(x)\in I \}$.

$\displaystyle p_+$ is the fixed point of $\displaystyle Q_c$ in $\displaystyle [0,2]$.

$\displaystyle p_+=\frac{1+ \sqrt{1-4c}}{2}$

I dont understand why it suffices to find the c-values the hint points to guarantee the conclusion for $\displaystyle x \in [0,\frac{1}{2}]$

thank you!

Something seems to be missing in the question

Tonio
• Feb 21st 2010, 06:57 PM
mabruka
hmm ok.

We need to prove that $\displaystyle |Q^{\prime}_c(x)| > 1$ for all $\displaystyle x \ in [-2,2]$.

It is easy to see that it is true for $\displaystyle x > \frac{1}{2}$

To prove it for$\displaystyle x \leq \frac{1}{2}$ it is suggested to investigate which values of c
satisfy:

$\displaystyle Q_c(\frac{1}{2})< -p_+$

My question is:
Why it suffices to find the values of c which satisfy $\displaystyle Q_c(\frac{1}{2})< -p_+$ to show that $\displaystyle |Q^{\prime}_c(x)| > 1$ for $\displaystyle x \leq \frac{1}{2}$ ?

In other words, why does the hint "works" ?