Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Possible Duplicate:
Show that $d$ is a metric on $\mathbb{C^n}$

On $\mathbb{C^n}$, define $||z||=(\sum_{j=1}^{n}|z_{j}|^{2})^{1/2}$ and for $z,w\in\mathbb{C^n}$ define $d(z,w)=||z-w||$. Show that $d$ is a metric on $\mathbb{C^n}$.

My attempt:

(1) (nonnegativity) It is clear that for any $z,w\in\mathbb{C^n}$, $d(z,w)=||z-w|| = (\sum_{j=1}^{n}|z_{j}-w_{j}|^{2})^{1/2}\geq 0$ since $|z_{j}-w_{j}|^{2}\geq0$. Also, $||z-w||=0$ iff $(\sum_{j=1}^{n}|z_{j}-w_{j}|^{2})^{1/2}= 0$ iff $z=w$.

(2) (symmetry) $d(z,w)=||z-w|| = (\sum_{j=1}^{n}|z_{j}-w_{j}|^{2})^{1/2}=(\sum_{j=1}^{n}|w_{j}-z_{j}|^{2})^{1/2}=d(w,z)$ by properties of modulus in $\mathbb{C^n}$.

(3) (triangle inequality) $\forall w,z,v\in\mathbb{C^n}$, $$\begin{align*}d(z,w)&=||z-w||\\ &= \left(\sum_{j=1}^{n}|z_{j}-w_{j}|^{2}\right)^{1/2}\\ &=\left(\sum_{j=1}^{n}|z_{j}+v_{j}-v_{j}-w_{j}|^{2}\right)^{1/2}\\ &=\left(\sum_{j=1}^{n}|(z_{j}-v_{j})+(v_{j}-w_{j})|^{2}\right)^{1/2}\leq... \end{align*}$$

I'm not sure how to split up the sum here.

share|cite|improve this question

marked as duplicate by Nate Eldredge, Srivatsan, Zev Chonoles Jan 28 '12 at 4:00

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

You wrote an equality in (3) instead of an inequality. Anyway, $\mathbb{C}^n\cong\mathbb{R}^{2n}$ as additive groups, so you can just inherit the triangle inequality from the general Euclidean case (or at least transplant the proof of it). – anon Jan 27 '12 at 22:09
well...that's embarrassing :P – Emir Jan 27 '12 at 22:42

If we consider a complex number as a pair of real numbers, then your metric is equivalent to the Euclidean distance in $\mathbb{R}^{2n}$

share|cite|improve this answer

Part (3) follows from Cauchy-Schwarz inequality. $\vert \langle z, w\rangle\vert^2\leq \langle z,z\rangle \langle w,w\rangle $

share|cite|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.