ucs clashes with DeclareUnicodeChar?

Question

I'm forced to work with latex, and want to make use of unicode chars in math. So I write:

\documentclass{article}
\usepackage[utf8]{inputenc}
\DeclareUnicodeCharacter{B7}{\cdot}
\usepackage[mathletters]{ucs}
\begin{document}
$a · b$
\end{document}

But it gives:

aΔb

Is there a way to DeclareUnicodeChars after ucs?

---

The ucs manual has something to say about DeclareUnicodeChars:

Several options have a special hardcoded meaning:
...
Every character you define in your document using \DeclareUnicodeCharacter
has the option document. This option is activated per
default having the priority 1000, therefore manually declared characters take
precedence over all other characters, unless some other option has explicitly
gotten a higher priority.
...

So I tried the way I understood it:

\DeclareUnicodeCharacter[document]{B7}{\cdot}

and

\DeclareUnicodeCharacter{B7}[document]{\cdot}

but it doesn't work.

---

Also tried:

\DeclareUnicodeOption{document}

and it also doesn't work.

Edit:

\documentclass{article}

\usepackage[mathletters]{ucs}

\usepackage[utf8x]{inputenc}

\begin{document}
\DeclareUnicodeCharacter{00B7}{\cdot} % U+00B7 = cdot
$a · b$
\end{document}

Gives:

! Missing = inserted for \ifnum.
<to be read again> 
                   B
l.10 \DeclareUnicodeCharacter{00B7}{\cdot}
                                           % U+00B7 = cdot

Also after reading ucs sources I tried:

\documentclass{article}

\usepackage[mathletters]{ucs}

\usepackage[utf8x]{inputenc}

\uc@dclc{183}{mathletters}{\ensuremath{\cdot}}%·

\begin{document}
$a · b$
\end{document}

but it gives:

! Undefined control sequence.
l.7 \uc
       @dclc{183}{mathletters}{\ensuremath{\cdot}}%·

Answer 1

Life is much easier without ucs:

\documentclass{article}
\usepackage[utf8]{inputenc}
\DeclareUnicodeCharacter{00B7}{\cdot}

\begin{document}
$a · b$
\end{document}

But with ucs you simply have to use the correct decimal:

\documentclass{article}
\usepackage[mathletters]{ucs}
\usepackage[utf8x]{inputenc}
\DeclareUnicodeCharacter{183}{\cdot}

\begin{document}
$a · b$
\end{document}

Answer 2

Life is even easier with newunicodechar:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{newunicodechar}

\newunicodechar{·}{\cdot}

\begin{document}
$a·b$
\end{document}

If you want to retain the default definition (\textperiodcentered) in text mode, then type

\newunicodechar{·}{\ifmmode\cdot\else\textperiodcentered\fi}

Answer 3

For the last example you could use makeatletter and makeatother. See latex2e.pdf for documentation of those commands.

The second code does not work anymore (I noted this after a release_upgrade from Ubuntu 18 to Ubuntu 20 with accordingly updated TeX versions)? \DeclareUnicodeCharacter seems to be an undefined contr[o]l sequence in this combo. Removing any of the three preamble lines compiles with warnings (and perhaps not the desired result)

Source: a comment

In such cases, this solution will still work.

Not that I recommend using this package (see package comparison: utf8x vs. utf8 (inputenc)), but there are some issues about normal utf8 that I'm figuring out.

%! TEX program = pdflatex
\documentclass{article}
\usepackage[mathletters]{ucs}
\usepackage[utf8x]{inputenc}
\makeatletter
\uc@dclc{183}{mathletters}{\ensuremath{\cdot}}%·
% use {default} instead of {mathletters} to make it work in e.g. text mode
\makeatother
\begin{document}
$a · b$
\end{document}