# Homework Help: Latex vector notation.

1. Sep 20, 2007

### IlyaZ

1. The problem statement, all variables and given/known data
a. How do I write vectors bold within displaymath and frac. I want Phi, r and r_1 bold. But get G M_1 bold as well = bad.
b. How do I use arrows instead of the bold notation?

3. The attempt at a solution
\begin{displaymath}
a1. textbf{\Phi(r)} = -\frac{G M_1}{| textbf{r}-textbf{r_1} |}
\end{displaymath}
a2. \bf{\Phi}(\bf{r}) = -\frac{G M_1}{| \bf{r}-\bf{r_1} |}

2. Sep 20, 2007

### Timo

Arrows: \vec : $$\vec \Phi ( \vec r) = -\frac{G M_1}{| \vec{r}-\vec r_1 |}$$
Bold: \mathbf : $$\mathbf{\Phi}( \mathbf{r}) = -\frac{G M_1}{| \mathbf{r}-\mathbf{r_1} |}$$

I don't think you really want the potential to be a vector, but that's what your code implied, so I kept it.

Last edited: Sep 20, 2007
3. Sep 20, 2007

### D H

Staff Emeritus
You get other bad stuff as well using \textbf. Don't use it in math mode.

\textbf{\Phi(r)} = -\frac{G M_1}{| \textbf{r}-\textbf{r_1} |}
$$\textbf{\Phi(r)} = -\frac{G M_1}{| \textbf{r}-\textbf{r_1} |}$$

\bf doesn't work at all:
\bf{\Phi}(\bf{r}) = -\frac{G M_1}{| \bf{r}-\bf{r_1} |}
$$\bf{\Phi}(\bf{r}) = -\frac{G M_1}{| \bf{r}-\bf{r_1} |}$$

Timo suggested \mathbf, but that doesn't work on some things.
If you have access to the AMSLaTeX macros, (and you should), use \boldsymbol.

\boldsymbol{\Phi}(\boldsymbol{r}) = -\frac{G M_1}{| \boldsymbol{r}-\boldsymbol{r}_1 |}
$$\boldsymbol{\Phi}(\boldsymbol{r}) = -\frac{G M_1}{| \boldsymbol{r}-\boldsymbol{r}_1 |}$$

Use \vec.

An even better solution is to define your own vector macro that formats vectors the way you want them to appear. For example,
\newcommand{\vect}[1]{\boldsymbol{#1}}

Then when someone tells you to format your vectors with arrows instead you can simply change that one macro.
\newcommand{\vect}[1]{\vec{#1}}
Bold symbols are used for things other than vectors (matrices, for example). A global search-and-replace is not a good idea.

4. Sep 20, 2007

### IlyaZ

Thanks, that works great!