From your question I take, that unicode is favorable to you. In that case, you might want to use the xetex or luatex engines which natively support unicode input.

Unicode has only one codepoint (u+2116) for the numero sign, which should only be used for cyrillic and asian, according to wikipedia, and incorporates a capital N.

According to the same source, it's Italian practice to write Numero as n+superscript lowercase underlined o, which can be replaced by "n." or n+Ordinal symbol , which is part of unicode as well: u+00AB.

Your source seems to mix these two replacement versions.

The following code shows the different possibilities (Numero sign, n.+Ordinal sign, n.+superscript o, n.+degree symbol, which would be the xetex equivalent of the \circ proposed in the comments) using the fonts TeX Gyre Pagella and STIXGeneral via `xelatex`

:

```
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\setmainfont{TeX Gyre Pagella}
№ n.\kern-0.5bpº n.\kern-0.5bp\textsuperscript{o} n.\kern-0.5bp°
\setmainfont{STIXGeneral}
№ n.\kern-0.5bpº n.\kern-0.5bp\textsuperscript{o} n.\kern-0.5bp°
\end{document}
```

I added the kerning, because the o/ordinal symbol in the original source seems to be horizontally closer to the . than in the produced copy.

The result shows, that STIXGeneral uses an Ordinal sign without underlining, which seems to be the most appropriate to me. But this depends on the font as TeX Gyre Pagella uses an underlined Ordinal sign.

So n.+superscript o seems to be the most practical way to go.

Simply add the lines

```
\DeclareSymbolFontAlphabet{\mathbb}{AMSb}
\DeclareSymbolFontAlphabet{\mathbbl}{bbold}
```

in your preamble and use `\mathbbl`

instead of `\mathbb`

for lowercase latin letters.

MWE

```
\documentclass{article}
\usepackage[bbgreekl]{mathbbol}
\usepackage{amsfonts}
\DeclareSymbolFontAlphabet{\mathbb}{AMSb}
\DeclareSymbolFontAlphabet{\mathbbl}{bbold}
\begin{document}
$\mu = \bbnu(a,b,c)$
$\mathbb{K}$
$\mathbbl{k}$
\end{document}
```

The reason why simply loading `amsfonts`

doesn't work is that in `amsfonts.sty`

you have

```
\@ifundefined{mathbb}{%
\DeclareSymbolFontAlphabet{\mathbb}{AMSb}%
}{}
```

which means to set the `\mathbb`

alphabet to `AMSb`

only if it has not been defined.

But it has been already defined by `mathbbol`

to be

```
\DeclareSymbolFontAlphabet{\mathbb}{bbold}
```

so it doesn't work without that additional line.

## Best Answer

After a little searching on the internet, I found the

`bbm`

package that produces the blackboard symbol for lowercase letter