\documentclass[letterpaper,12pt]{article}

\usepackage[dvips]{color}
\usepackage{amsmath,amscd,amsfonts}
\usepackage{bm}
\newcommand{\bvec}[1]{\ensuremath{\bm{#1}}}

\begin{document}

\textcolor{blue}{this sentence is blue except for \textcolor{red}{one} word?}


some text $\alpha+\bvec{\beta}$ is here.

some text $\beta+\bvec{\alpha}$ is here.


\vspace{0.5in}


Before the change, \verb+\phi+ gives $\phi$, and \verb+\varphi+ gives
$\varphi$.

\let\oldphi\phi
\def\phi{\varphi}

After the change, \verb+\phi+ gives $\phi$, \verb+\varphi+ gives $\varphi$, and
\verb+\oldphi+ gives $\oldphi$.


\vspace{0.5in}


One variety is \~\ $10.345$ for you.

One variety is \~{ }$10.345$ for you.

One variety is \~{}$10.345$ for you.

One variety is \verb+~+$10.345$ for you.

One variety is \texttt{\~{ }}$10.345$ for you.

One variety is \texttt{\~{}}$10.345$ for you.

One variety is $\tilde{}10.345$ for you.

One variety is $\tilde{ }10.345$ for you.

One variety is $\sim10.345$ for you.

One variety is $\approx10.345$ for you.

One variety is $\sim\!10.345$ for you.

One variety is $\approx\!10.345$ for you.

One variety is $\sim\!\!10.345$ for you.

\end{document}
