An inequality

• Mar 28th 2009, 12:40 PM
bkarpuz
An inequality
Prove that $\displaystyle |\mathrm{Re}(z)|+|\mathrm{Im}(z)|\leq\sqrt{2}\|z\|$ for all $\displaystyle z\in\mathbb{C}$.

Not very hard, but I wonder to see if there will be nice solutions.
• Mar 28th 2009, 12:51 PM
NonCommAlg
Quote:

Originally Posted by bkarpuz
Prove that $\displaystyle |\mathrm{Re}(z)|+|\mathrm{Im}(z)|\leq\sqrt{2}\|z\|$ for all $\displaystyle z\in\mathbb{C}$.

Not very hard, but I wonder to see if there will be nice solutions.

$\displaystyle a^2 + b^2 \geq 2|a||b|$ is equivalent to $\displaystyle (|a|+|b|)^2 \leq 2(a^2+b^2).$
• Mar 28th 2009, 12:59 PM
bkarpuz
Quote:

Originally Posted by NonCommAlg
$\displaystyle a^2 + b^2 \geq 2|a||b|$ is equivalent to $\displaystyle (|a|+|b|)^2 \leq 2(a^2+b^2).$

Of course this is the most simple proof.

And still waiting for interesting solutions.
• Mar 28th 2009, 05:17 PM
halbard
If $\displaystyle z=r(\cos\theta+i\sin\theta)\mbox{ with } r=|z|$ then

$\displaystyle |\mathrm{Re}(z)|+|\mathrm{Im}(z)|=r(|\cos\theta|+| \sin\theta|)=r\sqrt{1+|\sin 2\theta|}\leq r\surd2$.
• Mar 29th 2009, 01:14 PM
bkarpuz
Quote:

Originally Posted by halbard
If $\displaystyle z=r(\cos\theta+i\sin\theta)\mbox{ with } r=|z|$ then

$\displaystyle |\mathrm{Re}(z)|+|\mathrm{Im}(z)|=r(|\cos\theta|+| \sin\theta|)=r\sqrt{1+|\sin 2\theta|}\leq r\surd2$.

My own solution was also similar.
Just set $\displaystyle f(t):=|t|+|\sqrt{1-t^{2}}|$ for $\displaystyle t\in[-1,1]$, and show that $\displaystyle f(t)\leq f(\sqrt{2}/2)=\sqrt{2}$ for all $\displaystyle t\in[0,1]$.
Since $\displaystyle f$ is even, we have $\displaystyle f(\cos(t))\leq\sqrt{2}$ for all $\displaystyle t\in[0,2\pi]$.